When I did my kitchens, Home Depot quoted me a price about 3x that of the cabinets for installation ($5000 worth of cabinets too!), they said "oh we need to get permits, etc etc" bullshi'ite! They just want to maximize their profits while they subcontract out to the lowest bidder.
The cabinets themselves are not terribly hard to install, screw a board to the wall that's level and the height of your cabinets, then just rest them all on top of that, and screw them into the wall, then remove the board. You will need at least another set of hands to do it though.
For the countertop, yeah let them install it, great thing about HD for that is their prices per sqft of granite are installed prices.