the max size rims you can put on a natro skyline and the max size you should put on to not look like a cock are 2 different things. for a natro, stick to 17" rims.
what sort of spoiler are you wanting? some big stupid ricer style drift wing? if yes, then don't. just don't. they are bad enough on turbo skylines and even worse on natro ones.
as for bayside blue, the cost of a decent respray will be a few thousand dollars, depending on whether you get in the engine bay, etc done. certainly not worth it on a NA unless you plan on keeping it once you get off your P's.
just keep hunting till you can find a bayside blue natro, or simply buy another colour until you get off your P's