I think you are making this more complicated than it needs to be.

If you want a randonneur/brevet bike, figure out what used bike would be suitable, buy one, and overhaul it, restore it, modify it, maintain it. You will rebuild hubs and bottom brackets and headsets, replace housings and cables and pads, adjust derailleurs, clean and lube chains, true or rebuild wheels, etc. By the time you are done, you will have learned a fair bit about bike repair and will have the bike you want.

I would not do what you are doing, which seems to be to buy new or used bikes, assemble them or fix them, and give them away or sell them. The money you are spending would be better used to buy the bicycle-specific tools you'll need in your workshop. Plus, I wouldn't want someone else riding a bike I built/fixed, until I was pretty experienced in building/fixing bikes.