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.

Oh, here's a pic of the cockpit from the old bike, if it's of any help. Cheers.

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Wheels, if they were standard single speed rear hub and not a bastardised road geared hubs should be fine as steamroller is 120 mm at rear. According to surly the break reach is 47-57 mm so you need to measure yours but you may / should be ok. I am presuming surly use a standard recessed nut like your bike
Bottom bracket will depend on chain line on surly and what chain set you have. - probably your most fiddly problem!
Headsets are cheap unless it’s a quality one I would just get a new one

1. Most SS or FG track frames are 120mm wide at the back. Most flip flop hubs are 120mm, but some are now 130mm. Check what the Steamroller is. If it's 130mm and your hub is 120mm, you might have enough thread on the axle to just spacer it out behind the bearing lock nuts or put a longer axle in.
   
2. BB usually has a label on the barrel saying what it is i.e. type of taper/spline and width. Just replace it like for like. It should fit in either frame, assuming the Steamroller has a threaded bottom bracket and not a press fit.
   
3. Headsets can be taken out and re-used no problem. Your current one is 1" threaded, however, and the Steamroller looks like it has a 1-1/8" threadless, so it won't switch over. This may mean you won't be able to use your current stem or handlebars if you were intending to.
   
4. Your brake should be ok but you'll only know when you offer it up.
   
   

HTH

Cheers, this was really helpful! Definitely feeling most intimidated by the chainline but there is a great LBS nearby I can ask for help if things get too fiddly.

This was super, super helpful, thank you 🙏🏻

Nb. Very much on the lookout for a 56cm Steamroller frame for this build.