HP D30's aren't hard to find in the junkyard.....lots of XJ's around.
$100 is the going rate for a HPD30. I paid about $90 at the junkyard. It sat on my garage floor for months, while i acquired the parts necessary. I didn't swap it in until I finished installing a truss, reinforcing all the mounts, new ball joints and Vanco brakes. I drove it the alignment shop then the gear guy. I'd suggest you do the same....don't install it until you have EVERYTHING to make it happen.