8FUN Bafang kit as supplied by http://www.8funbike.com/detail.asp/sku=8F26F36B is recommended. 4.5 stars out of 5.
I live in hilly country, and at the top of the steepest hill in town. I've already twice had heart surgery, so it was a choice between giving up cycling and getting assistance. Fitting an electric motor isn't a big deal if you buy the right kit, but unless you are a skilled mechanic, and good with electronics too, putting a kit together yourself from different sources is not advised.
Proceeding from the assumption that you want to keep your Rohloff gearbox in the rear wheel and thus want a front wheel pedelec kit, forgive the abrupt sound of a schoolroom but there is quite a bit of knowledge to impart.
The best value kit is the 8FUN, found at http://www.8funbike.com/detail.asp/sku=8F26F36B
(thanks Dan, saves me looking it up again); many of the other good kits are just the same bits for more money. The two keys of this kit are the Bafang QSWXK motor, in which the Q stands for additional quality control to European standards, and the bottle battery, using special Panasonic cells, which seems more capable than its rating might suggest (and which is sold in Switzerland for much more than the entire kit costs from 8funbike). I found the British vendors of this complete kit to be good people to deal with. The Chinese-made instructions supplied with the kit are stupid; you have to ask the vendor for the full colour plain English instructions they have on their computer which they'll copy to you on request. It is very important that if you get any other motor/other kit, you ask what the diameter of the axle is, because the standard electric motor axle in the Far East is larger than the dropouts in your Thorn fork. If you don't want to file your fork, take this kit, or build up a kit with this motor, or make sure whichever motor you choose has the correct axle size so you don't have to file your fork. If you buy another kit, be certain to ask whether it comes with anti-rotation axle blocks because some expect you to provide your own torque washers (!). A firm called BMS Battery in China supplies kits without the battery, which you buy separately, also from them, but it works out more expensive by the time you've paid carriage, and you run the risk of additional customs charges, but you can have a choice of motors. A tempting motor stocked by BMS is the Bafang BPM "Climber", but it is illegal in Europe (over 250W average output), and my remarks about axle size may or may not apply. (There's an expert around here somewhere -- help!) The SWXK is known as the "Little BPM" for its torque rating, which is why I chose it. (Top speed is limited by any legal controller to 25kph, so you may as well choose from whatever is available the motor that works best on hills.)
If you buy the suggested kit, I don't see why you shouldn't fit it yourself. I fitted mine by simply turning the bike upside down, not even putting it on the work stand. You need to fiddle with the wheel a bit before the axle slips past the lawyer's lips, and you MUST mind that the anti-rotation blocks are well seated (their prong is a bit shallow), or the motor will rotate and rip out all the wires, but that's the trickiest bit of the assembly. There's a small problem, in that the wiring for the controls were made for a rear motor, on the assumption that the control box would be fitted on the seat tube. So one of the wires, for the pedelec assistance, is short and needs lengthening, and some of the other wires are too long if you fit the control box in the logical place for a front drive, the head tube. I've just got them all bundled into a leather saddlebag strapped to the handlebars for the time being, while the whole wiring thing waits for me to fit a BUMM front light made especially for using with a 36V battery (the motor sits in the place where the SON hub generator once provided power...). It looks a bit of a mess with so many wires not yet tied down, but I've ridden it for months like that without mishap.
The pedal assistance, which makes the bike a "pedelec", is stupid. The slower you pedal, up a hill for instance, the less assistance it provides. This is contrary to what is required. If you're a spinner, you can mitigate this undesirable effect. I'm a masher, so I use the the supplied thumb throttle instead.
Now that I'm used to the motor, I've been wondering if I shouldn't step up from the very light 38x16 gearing I have on my bike (a Utopia Kranich with 700C wheels and Big Apple 60mm tyres, 745mm rolling diameter, huge) to something a bit faster, say 44x16, because I no longer ride in 11th 1:1 gear but in 14th, just applying a touch of electric at small bumps in the road rather than gearing down, and gearing down below 8th gear only for very sharp, short hills, or massive acceleration at busy crossings. But the pedalpals think I should just slow down, because they're not getting younger either, and they feel I'm going faster now on average than before.
Under 9 Amp hours may not sound huge, but these particular Panasonic cells in the bottle that comes with the kit are pretty good. My standard ride is about 17km or ten miles of hilly country, and I use the motor about half the time but mostly at a part-setting, rather than full out, with short but strenuous full power use to get home up the last steep hill. On this and longer rides to over 30km, the battery has never indicated less than 50% available, and always recovered to 100% before the bike was parked again; it certainly has never run fully flat. A rule of thumb for using the bike like an electric vehicle rather than an electricity-assisted
vehicle is one mile per Amp hour but I've done more than nine miles on the flat at full power and the battery didn't give out, or take unduly long to recharge. Being a technonerd, I considered buying a meter to tell me about the innards of the battery and never got around to it because the thing just seems to work and work, plug and play. (And the 8FUN kit comes with a particularly nice locking aero-plug for the battery.)
The half a star deducted is for the one wire which is too short for the logical fitment of a front drive kit, and for a wire on a plug that was misplaced, and had to be tracked with a continuity meter and resoldered, standard tacky Chinese quality control in the tiny, irritating things. (Only the motor has the special quality control for the European market.) That it isn't a whole star deducted is because a) the actual fault is singular and very minor, five minutes of my time for a routine check I conduct anyway on any Chinese electronics, and b) the nearest competitive kit is over 60% more expensive for the same bits, without any guarantee of better quality control in the peripherals. At the current stage of pedelecs, having to use a continuity meter and a soldering iron is par for the course until you get into the Canadian Bionx price range (several multiples of the 8FUN kit...).
Hope this helps.
PS. Don't be tempted to buy the kit with the rack and the flatpack battery rather than the one with the bottle battery. It isn't worth saving either the few pounds or the water bottle mountings to unbalance your bike by putting the battery way out the back. The battery is atrociously heavy, and you absolutely must mount it in the centre of your bike, or even on the head tube, rather than on a rack out the back. There isn't in fact a kit for 700C bikes with the bottle battery, so I asked the vendor to give me a bottle battery from a 26in kit rather than the rack and flatpack battery that normally comes with 700C kits, which they instantly agreed to do, and without additional charge.