[400TT]

Tyre bead is what you call the edges of the tyre where it sits into the rim.

So if you measure a rim on the inside, where the tyre covers, on both inside edges, an 8" wide rim will measure 8". If you measure from far outside edges of the rim(the bit you scrape on the gutter), and 8" rim will measure 9". It's these outside edges that are used for back space and front space.

Personally I would buy rims last. That's what we have always done. There are always new styles coming out and you taste & requirements may change by the time you finish your build. Plus saves you having to store them safely.

Yep, it's a good idea to verify the offsets yourself and you may like your wheels sitting in/out a little further than what someone else does. It makes it a lot easier to measure wheel offsets etc if you can borrow some wheels that are close to what you want, so see if you can bribe someone local.