Doubt those VVC's have done 20k, and wouldn't pay a grand for one either way. It's better to hear the engine running if you can, but not always possible. I've bought engines from scrapyards and some have ran spot on, but others were shot. I bought a VVC for a member on here awhile ago and was told it ran fine, only for him to fit it and finding out it had 2 holes in the pistons making it scrap. Some other 30 days warranty but don't mean shit. The engine support (mounts) need swapping from the timing belt end from the metro's engine onto the 1.8, and only that if your staying R65.

I like the R65 in a metro strapped to a VVC, it makes it accelerate a lot quicker than a PG1. Most will say it depends what ratios the PG1 has, but I've tried a few and think the R65 is still the better box. I used a 1.6 flywheel from a Rover 400, but most rover 1.6's use the same size, so a R200 400 etc will do, but make sure it's a 4 pole, and use a 200mil clutch. I believe LSD's are very rare and hard to find for the R65. If you go PG1 you'll need a mgf/tf subframe 'modified' and shafts, engine spacers, selector rods etc like you said.. 

Wiring isn't my thing, but AdrianSi and a couple of others on here can sort you a loom, but they all use facebook shit these days, so maybe better off contacting them on there. You don't need the fusebox from the donor car, just the fob/s ecu and 5as with the right mfu. The 5as immobiliser is better than the one you have now. 

You use the alternator on the 1.8 with the metros belt, after removing the aircon pump etc from the 1.8. Starter motor depends what gearbox you plan to use. Use the Metro's starter if your keeping the R65, or fit one from a 1.6 rover as its a little stronger. Or if your fitting a PG1, you'll need a PG1 starter motor. The fuel pump is fine as it is as it's already a MPi fuel pump. Ignition coil isn't needed as the VVC has coil packs on top of the engine. 

Most people fit a MGF rad or a derv, but I used my GTi's rad as it was mint and didn't have any cooling problems with a 82 stat.

You can use the manifold and exhaust your using now, but to get the best power use a VVC manifold modded to fit. Easy enough, just weld a sections onto the down pipe.

Someone with more info may pop in and tell you more if they can get off facebook and away from the online friends