Should I rebuild or keep fixing

What should I do? Should I keep doing little fixes or do a full rebuild?

I took out my diff and it's a total wreck...will get a new one. Whilst doing that I noticed the hub bearings are gone...took off the hub and the stub axle is badly worn. Now I'm thinking whether to not just rebuild the whole axle, powder coat and all.

Same goes for the whole car...chassis is sound but I'd love to have it galvanised.

Even if you do rebuild I wouldn’t powdercoat it. I find powder coating doesn’t last that long and hides lots of problems. Water can track underneath it and cause corrosion but it will still look perfect. The first thing you know of it is when a whole sheet of the powder coat falls off and reveals there is not steel left underneath (axle will be too thick for this to happen with). You would be much better with a suitable chassis paint.

As to rebuild of fix. Other than the painting bit which you do not need to strip it to do you sound like you have are already replacing all the wearable parts anyway. So a rebuild does not sound necessary.