DM8 and A1 = High End
Mini and PMR = mid end ish?
If you are comfortable buying second hand then i would go for an A1 or DM8 (there are also plenty of Etek 2s hanging around in your price range)
The Invert Mini is the cheapest both new and second hand and i would say it is the worst of the 4.
I can personally vouch for the PMR 09 being an owner. Its a great, fast, light, consistent gun that looks decent and can easily hang with the big boys. It is however still a simple low-end design being an unbalanced spool valve (meaning the bolt is only moved in one direction by the solenoid it is moved forward by the pressure in the dump chamber - unlike a Smart parts Shocker where the bolt is pushed in both directions) It is also a gas hog (i think somewhere in the region of 700-1000 shots of a 68/4500 tank - unlike the Angel which will get 1500+ i think)
But the PMR also uses the same board that you will find in the DM8 and DM8 with the same style grip and trigger. It also has a nice barrel and a nice on/off ASA that the Mini doesn't have.
Basically you are looking at a new vs. used debate as you can buy the Mini or the PMR new within your budget but the DM8 and A1 would be used.
Personally though... i would have the DM8 or the A1.