Stay away from the XB's. You can not get any parts for them. Get a "tuber" (tube frame) that has a Harley Sportster engine in it. You will always be able to get powertrian parts from Harley and aftermarket, the rest of the parts are easy to find online.
Buells are worth more as parts donors that whole bikes. You can buy a nice XB for 4k and sell the parts on ebay for 8k.
My all time favorite Buell is the X1 Lightning. Fast and powerful, 100 hp out of the box that can be brought up to 125+ and a tube frame that can be straightened if you happen to go into a corner a little to fast.
My son bought a cheap used 883 Sportster and I put all the X1 parts in the engine and a Mikuni carb. It flys now.