Polyurethane will work on the neck. It will yellow over time, but some people will do a yellow stain to make it look more vintage straight away. It cleans off the frets no problems. Most products will as it quite difficult to get any paint to stick to metal without good prep work.

Yes it looks like you will need to drill a bridge earth hole, just make sure you mark where the bridge is first and drill from the top of the guitar into the cavity.

320 grit is fine as you are going to need to prime the surface to do the solid colour. Be patient allowing the primer to cure as if you don't it will sink back and show the grain later. This is also a good time to make sure all the main surfaces are super flat, any wobbles will be very evident in a gloss black finish. My technique in this thread:

https://www.buildyourownguitar.com.a...ead.php?t=9001

For high gloss black I would use automotive acrylic. You can get 400g Septone gloss black from Supercheap. Build up a number of coats over a couple of weeks. Let it cure and harden as long as possible. With your nose right up on the surface of the guitar you shouldn't be able to smell any solvent. Putting a fan on it once it's tacked off is a good way to evacuate solvent from the surface and speed the process up a bit. Then you need block it back with a firm flat block and wet and dry sandpaper, being extremely careful not to get any moisture in any holes or cavities (the wood will swell and crack the paint). I normally go from 800-2000grit avoiding any hard edges as they are very easy to break through. Then using an electric buffer ideally, cutting compound, then cut and polish, then swirl remover and you should end up with something like this:



Be warned, high gloss black is one of the most difficult finishes to get looking good.