The X-cab on a short bed frame makes a nice platform to build from. For $2k it is a good deal, maybe offer $1800 because it doesn't have larger tires, matching paint, or a finished flatbed. You should be able to fab up a useable flatbed for a few hundred dollars, after shortening the existing base to fit the frame better.

If you do get the truck it will run better with a cat, these engines need some amount of back pressure, and have been designed to work with a cat, not to mention helping to keep the air cleaner.

As for the difference in performance, the 22re will be slower unless your 3.0 is only running on 4 cyls, or you have very low compression. I have both a 3.0 X-cab and a 22re X-cab. The both rigs have been geared for the tires they are running, the 3.0 is mod'd but weighs a good 1500lbs more than the 22re rig, yet ot will run circles around it all day. With the 3.0 loaded with gear/passengers weighing about 5300lbs I can climb the Sierra's at 70mph, however with the 22re loaded with the same gear/passengers I can barely get over 50mph on the steep grades, the same is true for others I wheel with that have the 22re and X-cabs. There is quite a difference between the two engines and transmissions. My 3.0 is fun to drive, while the 22re just gets me around from one place to another.:o