If you're buying a euro car motor, (and why not, Europeans are pretty good at making small car motors)
then I'd really recommend getting a peugeot/Citroen motor. the reason I say this is because not only are there haynes manuals, but peugeot do a website called service box, where you can sign up and get exploded parts diagrams of every part of the car.
Additionally, you can get the engine management cable and software (though not exactly official) through ebay, (search for lexia), it's also possible to find a service box or sedre download where literally everything wiring diagrams and all are available online. but packaged up into zip folders.
the software for working these is pretty good, rather than just giving you p-codes which can be pretty generic you get a very specific error messages.
Additionally, if you get an older one, (based on the EDC15 ECU) they are pretty well documented now so that you should be able to modify the ECU to either tune to get more power, or tune for emissions to help reach your goal...
(if you do this then look for the DW8 (1.4 diesel) or the DW10 (2.0 diesel version of the engine, -these are the basic diesel engines, pretty bullet proof and used in most peugeots and citroeons, (from small hatches/compacts up through their light trucks and vans).
the dw10 engine is the one that's in my car, and it's completely wire controlled, (e.g there is no cable on the pedal just a position sensor) so you'll find it easier to do the diesel electric thing of running the engine at a peak efficiency at all times, than if you have to worry about pulling a wire. -I'm assuming with a lower power engine than motor you'll be spending a lot of time running at less than full power as you need to wait for the battery to charge so that you can have a burst of speed? -hence running the engine at full power (even when stopped at lights etc) is preferable as this gives you the get up and go to leave the lights and come up to speed.
additionally these are the engines that the ford duratorque engines are based on , so getting parts for them probably won't be all that difficult, (though you could find that GM parts fit your opal engine?)
as I said earlier the older engines don't need quite so much with regards the body electronics, (for example no ECU to speak of, no BSI, just a pretty plan old engine turns and runs a mechanical pump for pressure and mechanical distributor for working the different injectors.)
it's not that I doubt the ability, more that certain things are critical, (e.g the engine won't run without ECU, BSI and dash board, and immobiliser satisfied (which can involve needing the steering wheel key and transponder etc) and those systems coded to each other... -this might mean that you have to have a lot of stuff in the truck that you wouldn't otherwise have wanted... and might not be useful...
(for example the speedo take off might need (must) have to be taken from the gear box, and additionally might need to have coherence with the signal sensed from the ABS sensors... or the system throws errors...)
then again, I'm biased, given that as I said earlier I already own a car with this engine, there might be resources available for vauxhall/opel cars that I'm not aware of, (as I haven't really looked).
