Welcome Rooftop from another Michigan guy! I've been riding 3-wheelers and quads since I was 12 yrs old - 38 years later I buy my first side-by-side. The guys on here are great- great depth of knowledge and experiences, as well as opinions! So with my 2 years of P500 experience here is what I say:
* A 26" tall tire is an excellent choice for this machine.
* What ever brand you buy - make sure it is a radial! Vastly improves the ride quality!
* Start with just the tires on your stock wheels (what I have) then if you think the machine feels too tippy- get some wheel spacers.
* As other point out- if you want to change the look and want the wider stance, buy a set of aftermarket off-set wheels.
* My experience is any aftermarket tire tread design is better than the stock tires than come on most ATV's & side-by-sides. So I would just say look at a bunch of different ones and pick what looks will suit your needs. As for a recommendation: I agree with the Maxis Big Horns- know a lot of guys that have run these on their 4x4 quads and the traction improvements are incredible. Or, I went with a bunch of suggestions from this site and bought the Kenda Bear Claw XTR's. They have a more aggressive tread than the Big Horns do, but I can accept the turf damage in my yard from them- not horrible. Oh- Big Horn 2.0 for the radials. I think the original Big Horns are bias-ply.