I'm looking at a staggered set of Rovos Durban 20x10 / 20/8.5 in gunmetal with nitto nt555's 255 in front and 285 in rear. Site says they fit my mustang (LMR). I don't know a lot about offset...my car has stock suspension but I will be adding Steeda Sport Springs, BMR Adjustable Panhard bar and strut mounts.

Will these fit as my car site now without any issues?

Are these the best size tires for this wheel? I like wide

 

You should be good on the Steeda Sports. Go any lower and you might rub in the rear, but it'll be close. I had 20x10.5 +40 on the rear of mine with 305/30 MPSS, H&R Supersports, and it rubbed slightly in the rear. Dropped down to 275s and had plenty of clearance. 295s might have been okay, 285s for sure.

But for your projected height and tire size, you should be good to go.