DMR's require a budget similar to a BASR. There isn't much shortcut or "easy buy" for this kind of weapon unless you simply have a "hot" M4.
Good on you for not going down the M4 with a phat spring path though.
Good contenders would be like an AK SR25 or Cyma CM98 however both will still need about the same again if not more going in to them to make them tolerate the additional power AND be accurate at range.
My personal DMR is a foooking long M4 with a Kythera HPA system so it's easy to get the big FPS if needed but even then I've swapped out all the hop and barrel set up as well to get the accuracy I need.
If you are "only" aiming for 400 fps, honestly I wouldn't bother, just make a "normal" AEG up to about 340fps and then concentrate on consistency, hop up and inner barrel as the difference really won't be that much, I was running my battle rifle this weekend with 0.3's and I was ranging the DMR guy (who was at 400) and I had full auto capability.