Lots of questions, I try to answer just few.
Converting to fretless, my advice: turn to a pro if you do not have decent tools. In a youtube video conversion may seem easy.
Madinter is a good company to source materials. I bought Blackwood to my fretless, but a pro installed it.
2.1 Cocktail sticks and glue is a tried and true way.
2.3 Painting, a good quality paint pays itself already in the beginning. 5 € bottle compared to a 30 € paint, there really is a reason for the price difference. If you buy two or three bottles from a pro paint shop where they mix it while waiting, the colour is the same. Some cheap bottle that has stayed in the shelf for months may be whatever.
3.1 If you are after a cheap solution, buy a bass from California Music, Thomann, or similar shop. Zikinf may be a good source to find used stuff.
Building is fun, but cheap it isn't.