I'm in the process of installing a Ledger board to my concrete foundation and I am using TAPCON screws. My 3/8" B&D drill will not drive the screws. I'm now thinking of using a hammer drill. Any thoughts as to which one I should purchase......How much tourque/power do I need?

Your 3/8 in drill should drive the screws in just fine provided you have drilled a pilot hole first. Don't think those screws are designed to bore into cement like a deck screw in wood. The hammer drill would only be used to drill the pilot holes and not to drive the screw in. Use a good carbide masonry bit.

Before you buy another one unless you just want a new toy

MAKE sure you had a carbide MASONARY bit
when you where drilling pilot holes

also try tiniest piloy hole you can make followed by a larger pilot hole

that should get the job done if not

weeeeeeeeee you get excuse to buy a new toy

I recommend a 18v hammerdrill of a brand of your choosing