Personally, I like some of the "kit guns" made with USA receivers over conversions that are based on 1mm receivers.
Here's what I would (and have) looked for:
1. 1.5mm or 1.6mm stamped steel receiver thickness. I have handled and shot 1mm, 1.5mm and 1.6 mm receiver guns, and I greatly prefer the fit, finish, quality and sturdiness of the thicker stamped receivers.
2. double check to make sure it is one of the known, quality receivers, and not something built out of an old shovel.
3. Parts are important. IMHO, the Polytech parts sets (must be built around a 1.6mm receiver) are the best. Very hard to find, however, and you might as well pay the premium for the Polytech pre-89 import. Next, I like the Polish parts, all matching, underfolder and chrome barrel. I have one of these and it is a good build. I still like the Polytech 1.6mm AK with slightly heavier barrel and more robust internal parts better, but my Polish parts gun is a great machine.
I bought my Polish underfolder built on a 1.5mm DCI receiver several years ago - all numbers match on the parts, and they look new. Hard to find these nowadays.
Here's a Polish Underfolder built on a DCI 1.5mm receiver with Tapco fire control parts for compliance. The Tapco trigger is actually pretty good much less slop than the Polytech I had.