I run in j-stock, so I'm not going to be able to give you much good info about IP cars in particular.

but:
Assuming your car isn't street-registered, you can get insurance through MotorsportCoverage.com for off-track stuff + storage.

Most IP cars are not registered for the street. It's doable, but probably not worth it in the end.

tires: you can always run Toyos, but you'll be giving up at least a second a lap to the guys on hoosiers.

IP is a really good class, and quite competitive. my advice is to buy a car that's already competitive in the class.

See here:
http://www.bmwccaclubracing.com/racers/Classifiedad_display.aspx

1988 BMW M3 #110 (NASA GTS2, BMWCCA JS)
2003 Expedition tow rig
1994 BMW 318i (commuter)
ex:
Audi Coupe Quattro
Porsche 911sc RoW
Honda CRX Si (x2)
1988 BMW M3
1988 BMW 325is
1991 Mazda Miata
1964 Dodge Dart - check out the rally blog

http://cars.g93.net