Lot's of people recommend Glock clones, but to me they're really not the ideal starter project ...
- Relatively expensive. A quality slide and barrel and OEM parts kits cost as much or more than just buying a good used Glock.
- Obviously not an option in places that restrict the sale of gun parts.
- Too easy to slap all the parts together with zero understanding of how the thing actually works.
- You don't learn much. You're printing one part and buying everything else.
My advice is start with the Decker 380 (https://guncadindex.com/detail/RELEASE-Decker-3.0.1:d). So cool to be able to post links! Low cost, easy to print (you can actually print every part without any supports), easy to understand, and quite innovative. You can buy a barrel (no need to ECM) but that's the only gun part you need -- everything else is hardware store material. The design is quite safe, and doesn't demand a lot of manufacturing skill or experience or any special tools.
It's also fun to shoot (the release even includes an optional designed-in Recoil Assisted Reset trigger) and a real attention grabber. I had a new development version at the range Friday for testing and the ROs were literally lining up to take pictures of it :)