Well, I'd like to be able to crank out a few car parts...
"I need a radio knob for my 1974 Buick - what do you mean they aren't available?"
And a few Glock mags, and a lower or three...
Ender 3 pro owner here. My personal upgrade/favorite is to use a glass build plate and generic hair spray on it for trouble-free adhesion, without too much adhesion. The hair spray works great as both an adhesive, and a release.
I did try printing PETG on bare clean glass, and that stuck so damn hard it tore chunks out of the glass.
You will want an enclosure around your printer if you want to print ABS etc. air currents, drafts etc. will cause layer separations and warpage if the printer isn't kept cozy, about 90-100F. I just bought the Creality "tent" enclosure for $80, rather than try to fab something on my own. But even a big cardboard box will work. Keeping the printer warm has other benefits, the build plate heater, and the nozzle/hotend don't have to work as hard, and won't cycle as often.
PLA will do fine in the open air.
Light use car parts should be no problem. Stuff like knobs etc. 3D printing is perfect. And of course you can use 3D prints as mold positives to cast more durable things in aluminum. Just make the 3D printed positive a few percent bigger for metal shrinkage. (I think you've got machine tools so could even clean up a casting?) You can even do thin mold positives with light minimal infill in PLA, and just let the molten metal burn it out directly when you pour.
Menendez/Extendez Glock mags work pretty well. I've printed several. Getting springs for them from the Glock store or Wolff is hit or miss because of everything being crazy, but you can always wind your own out of music wire. Some of mine are going strong after hundreds of rounds, some, the feed lips blew off just sitting on my work bench unloaded. I just printed a new one and reused the spring, baseplate and follower.
https://youtu.be/HLw6Yb9d1fAI mainly use them in my "Mac Daddy" build that uses a MAC upper and bolt, AR FCG, where the lower is 3D printed and uses Glock mags.
https://imgur.com/r/firearms/ReK8RusMost of the AR lower files floating around are pretty good. The main weakness is the buffer tube tower and threads, and various designs address it in different ways. You might want anti-roll pins for the FCG, but hell, like everything else, if they egg out on you, just print another.
I've printed and done AR builds on a "U-Bolt Vanguard" that uses a 1" U-bolt from the hardware store to reinforce the buffer tube tower, and there's newer designs out there with other reinforcements for the buffer tower.
I've also 3D printed a .32ACP VZ61 Scorpion lower receiver that uses a parts kit upper and bolt, Scorpion mags, and an AR FCG. It runs well.
https://youtube.com/shorts/Uvbuo24BWEg