M38s are fine machines if restored properly. Anytime you are making alterations to the frame (chasis) you are weakening it. Things to look for would be, the method used to lengthen the body. Was the frame cut, added and welded to increase the wheel base? or was the body only extended and the frame left intact? or was body extended with support added to the frame without cutting it?
If cutting/re-welding has been done on the frame, I wouldn't go for it. Since it be an off road primary. if the body was only lengthened, It can be brought back to original. So no problem here. BTW, leaving the extended back on will cause sagging of the extension sooner or later. It will help if support to such extension has been added, check the quality of work done. I would bring it back to original. but you can leave it as it in the last case.
Registration and tax papers should be without error for any vehicle you buy. it is mandatory. Goes especially for restored jeeps.
the rig is at a shop so work's already been done on it. the price would be high. if you buy it you are going to bring it to your taste later regardless. The total project cost will increase hence. If you wanna go for an older jeep, my suggestion would be to find a legit frame with ok body and all things. Build it to your likes and requirements. I believe it will cost you less and give u more off road pleasure. it will take a lot of sweat and patience though
that's just how think anyways..