Driving a Miata's *gotta* be preferable to anything that can pull a trailer carrying one. (Goldwing?) Besides, if you want to drag knee you can always open the door.
Factory wheels for 93 are 14x5.5 -- dunno 'bout offset. You can probably get away with running stock for now, but once you start kickin' butt the crybabies will force you to toe the line on regs.
If the vibe occurs around freeway speeds, it's a documented characteristic of (early) Miatas. Light weight, short wheelbase, open cockpit, and VERY sensitive to wheel roundness and balance. The last item might be the first thing you want to try.
Anyway, welcome! (And nice sig. Hope you don't mind if I borrow it!)