Mechanically, there was zip difference between the early ones and the late ones. Not like Radoms at all.
What you look for in an early gun is fit and finish, matching numnbers, same as any second-hand gun.
There are a few nice ones around. I scored one a few years ago: Mauser-Werke, 1944, nice-looking piece in 99% condition.
Original wartime mags are Waffenamt marked. P-1 mags work just fine and they are a lot cheaper. Original Walther mags with the banner trademark cost half the Earth and a mortgage on your virgin daughter, but they do exist. Did I mention that there are lotsa P-1 mags around... and they work just fine? You have the idea.
Original holster should be the black pigskin flap type, holds 1 spare mag. Original ammo was packed in 16-round boxes. P-38s don't really like a steady diet of Sten ammo but a box or so won't hurt them.
Late-war examples, especially those built by Erma-Werke at Dachau, really ought to be gone over VERY carefully. They were using labour from the KZ and some of the workers might not have had the greater glory of Fuhrer and Vaterland at heart.
Only real weakness in any of these is that silly little stamped-tin top action cover. It is flexible and just pressed into place. If you pry it out, you CAN get bits and pieces rather a remarkable distance away. It should fit closely and tight if you're intending on shooting the critter.
Check for cracks in the locking-block; that's where problems show up, generally, if they are going to show at all.
Have fun!