MGN is miniature, more suited really to lighter duty machines like 3D printers, laser etc

You want HG really, and HGH20 is a very common size which will be more than sufficient for your needs but easier to grease than HGH15 (15mm).

HGH20 is common on places like aliexpress and available fairly cheap, won't cost much different to MGN15.

As to that vs 12mm supported rail, well that's really up to you. HGH20 will be superior, but it will cost a little more. You have to do a cost benefit analysis on that yourself really... so to answer your question, your machine would definitely be better off with HGH20, but your wallet will be worse off.

Definitely go nema23, nema17's are good for 3D printers but don't have the power to do any decent milling imo and you'll end up chasing your tail and ruining stock with missed steps from motors that aren't up to the job.

Spindle wise, either a 1.5kw or 2kw water cooled spindle will do a good job and you can always move it across to a better/bigger machine later on if you want to. Given your relatively modest requirements there is the option to go cheap on this and use a dremel type router, but I find this to be a false economy as I went through 2 of them in short time on my first machine (and that was proxxon which supposedly had better bearings than dremel) before deciding it was pointless.