If you want a GOOD DMR, I would start with a good brand like realsword
1) FPS means very little in terms of range and accuracy
2) match your BB weight to your fps, 380fps = .28g BB, 470fps = .36g BB etc
3) range and accuracy means a high quality barrel, perfect compression & seal, and a high quality hop rubber
4) beyond 430mm or so, any extra barrel length is just a waste
I ran my marui VSR-10 at 380fps for a long time, and it had incredible range and accuracy. But that's a bolt action rifle and in every way mechanically superior to an AEG in terms of achieving perfect compression.