Which stroker kit is good for me?

Give Jim a call and tell him what your plans are, he will steer you in the right direction and not try to sell you anything you don't need. Tell him the guy from PR sent you.

why is it that the PRO STREET kit that handles 800hp is cheaper than the Street Fighter kit that handles 500hp? Did they make a mistake on their site?

If the pro street kit is really $1279 I would get that. You don't really need the forged crank that comes with it, but for some reason it shows it's cheaper. Basically, for what you're looking at, you would want to go with:

Cast crank
H-beam 4340 rods
Dished forged pistons
Approx 8.5:1 compression

With a SC down the road go for a forged crank combo, the SC puts a ton of stress on the snout of the crank, the forged will be less likely to flex and snap.

A good name brand forged crank and rod combo with ARP8740 cap screws, a decent piston should hold 800hp with no issues, if you want more then that or have some extra money upgrade to a ARP2000 rod bolt.