FWIW:
P38's are generally considered to be WW2 collectibles more than pistols you shoot for pleasure.
P1's Have alloy frames and are priced less, and are fun "plinkers". Early P1's had frame & slide cracking issues, but those were resolved by the addition of a "hex bolt" in the frame (for increased strength against stress), and what is called a "fat slide conversion" which consists of a slightly heavier slide, and (I think) better three white dot sights. (Not sure about the sights business.)
I HAVE read of alloy frame P38's, but never seen one, and my guess is that as the P38 swapped over to the P1 aluminum frame some were P38 marked.
I had a pristine P1 several years ago and it was surprising accurate at any reasonable distance and fully reliable. The majority of rounds were ball ammo for plinking, but I did run several mags of JHP's for fun, and it also fed them perfectly.
Just personal opinion, but if I wanted a WW2 collectible I would invest in a high quality P38 with all of the correct Germany markings, and if I wanted a pistol for shooting pleasure, or perhaps even to do double duty as a home defense firearm I'd buy a P1 with the hex bolt in the frame and the slide conversion.
www.gunparts.com, and
www.wolffsprings.com have parts available should you need any.
I think the Walther P5, which is actually the final version of the P38/P1 type platform is a fantastic pistol, but would cost more than a similar P1.. The P5 has a great reputation for being of top quality manufacture and is a nice option to the P38/P1 and will retain it's value (IMO) better.
Hope this helps.
Jesse