Poured concrete is a solid piece, whereas block is many pieces. The more pieces, the more places things can fail. If the house has a proper moisture barrier on the outside of the foundation, it shouldnt matter for moisture penetration, but again... one piece vs. many. I dont think I'd pass on a house soley because its block, unless a home inspector found something wrong with it.
Your studs go from the toe plate on the floor, to the header attached to your joists above, so it shouldnt matter what its made out of for framing walls. I wouldnt frame a wall directly to the foundation walls. When you frame the walls, you'll fill the stud bays with insulation, and you can put in whatever R value you want. A few feet underground, the temp is more or less constant anyways, so the basement shouldnt need much help from the HVAC. Make sure you do put in an air return in the basement when you finish it off. You'll want that air recirculated through the unit for humidity control.
