All you need are duck tape (if it moves when it shouldn't) and WD40 (if it doesn't move and should)

Seriously though, you seem fairly well covered, the addition of some rigging wire/terminals and a spare starter motor as already suggested would be a worthwhile. Remove the existing starter and take it to a decent auto-electrician, he will match you up with a perfect alternative to the manufacturers 'marine' original at half the price (or get a recon one for a quarter!) same goes for alternators too.

If you think the space/risk balance is hard on a boat, we travelled overland by motorbike a few years back - now that is a challenge.

Just accept that the bits that break will be those you didn't buy and don't worry about it; if they're cheap & small, buy two when you do track the part down.

From our experience, the biggest 'breakdown' items seem to be engines - not serious, if you have a rag and stick and refridgerators - try to repair before the beer gets warm or civilised life will come to an end.