The Mossberg 500 pump-action shotgun is an excellent choice for a home defense firearm. I think you'll find the regular stock much more comfortable to shoot than either the pistol grip or the folding stock, and it's still not a problem to maneuver indoors. Try to get to a gunshop or gunshow to handle one and see how it fits your body.
The standard 6-shot capacity, 18-inch barrel model would be fine for your needs; they make 8- and 9-shot models if you're so inclined.
Don't use any load smaller than Number 4 buckshot for self defense purposes. Slugs aren't necessary, and the No. 4 shot spread from an 18-inch barrel at indoor room ranges is negligable, so you still have to aim.
Prefectly serviceable used shotguns (think police trade-ins) are available at reasonable prices, probably less than $250.