There were a very few .25 PPK pistols made. A few collectors on here would certainly trade you a restricted .22 AND a restricted .380 for an original .25 PPK.
The nicest PP or PPK pistol to shoot, in my humble opinion, is the .22, I love those. There have been actual scientific studies done to determine why the .380 PPK has more felt recoil than any other pistol in its size range. These efforts are documented on Waltherforums dot com.
I have written reams about the difficulties mere mortals encounter when converting .32 PP or PPK pistols to .380 ... It's touching to see that someone remembers. The only reliable way to do this conversion is to cut the groove in the frame that Walther used post-war in their pistols, and change out the mags. The original ejector can be ground down to work with the new style mags.
Pre-war and wartime PP and PPK pistols in .380 are almost all deadly accurate, and all have bottom mag releases. Walther believed that it was impossible to build a reliable .380 pistol with the side mag release, and with their original design, they were right. Cut a groove in the frame and eliminate the ejector / slide lock bump, and hey presto! Reliability.
I have a few restricted barrels for the .22 PP/PPK and need to install and test a couple. I'll report back here, with a few pictures.
If you're reloading anyway, the 32 to 22 conversion (bottle necked, .223) is supposed to be a great little cartridge, and should also require only a barrel change. It's on my very long to-do list, but nowhere near the top.
Epps has done some very nice .30 Reid conversions, and are reasonable in their price.
My 4 inch model 28 is the most accurate revolver I own. It out shoots the 6 inch and 8 inch guns, and I would be really unhappy if the barrel got swapped out and the accuracy fell off. Sorry, I wouldn't trust that little gem to anyone, there are sad stories about each gunsmith out there, and there is no way I'd chance it. There will be plenty of space in my coffin for the model 28, my model 13, and a few others. In a safe, of course.