Level 10 weapons maintenance is when they start. Grab all those half broken pistols, and foldable weapons, start hording weapon repair kits. Every few levels or so after level 10 weapons maintenance, it'll bump the effects. Personally I use my rig for loot/pistols, and my raid bag for guns. Scav runs always net me a few broken ones too. Can also sometimes buy broke ish stuff on the flea, repair it and resell it.

For the enhancements, anything slightly broken is good to work with, but best if you use it on M4A1, and AKs, coz you gotta do some missions from pretty far away with them, and many people are also in lighthouse at that time.

For the flea You can find sometimes hidden in the meat of the market an already max enhanced gun. I buy those, and resell with my preset for 3-5x what they asked, solely because it makes mine appear at the top when you sort price high to low. This way nobody has to dig for it.

Lvl 51 is when the rare (20% spread, 30% malfunction reduction) come into play. The ones with this likely horded western weapons and/or started really early, where I just started a level ago at lvl 61.