Some dummy questions about road/SS builds

Hey, some advice please!

I'm living in Germany at the moment. Back in the UK I have a generic but fairly decent SS wheelset from an extremely beloved rat bike (a ~56cm road conversion), which sadly died a horrible death shortly before the move. I want to build up a nice sturdy Steamroller in its place; unfortunately however I can't afford to splash out near a grand (€, but still) on a complete new build from one of the LBSs here, and I'm also keen not to ditch perfectly usable parts if I don't have to. So, tl;dr, I'm thinking about getting as many of the components from a rat bike over to a new Steamroller frame. However I'm still a novice in lots of areas with a lot to learn about component standards, so if you'll indulge me, here are a few probably pretty dumb questions before I go to the hassle of bringing parts over in person:

1. Are hubs for 700C wheels a universal size? ie. if the rat bike took 700c wheels, will that wheelset definitely fit a newer bike, or is it not so straightforward?
   
2. The BB on the rat bike (square taper, from what I've gathered) was on its last legs, so that can go. I have its crankset and chainring over here with me already; if I were to get the wheelset over, and they fitted the new bike, would I just need to have a new BB of the same type as the old one installed in the new frame (or again, is it not so straightforward)?
   
3. Can headsets be rescued and transplanted onto newer bikes, or would that be something really worth getting new for a new frame?
   
4. Are brake calipers also pretty universal? The set I had on the rat bike were perfectly good and had a lot of clearance, and I would guess they'd fit onto the new one but again, I could be wrong.
   
   

Any advice from more experienced bike builders will be extremely appreciated! Viele gruße.