No guns are good enough to be a DMR straight out of the box, they need a lot of upgrades, just like a bolt action sniper rifle.
Some components are more obvious than others to replace (namely hop unit & bucking, barrel) but preferably you want to install a high torque motor, re-shim / AOE correction / lock to semi work etc
Therefore internal parts in my knowledge aren't that much of a problem since you're actually gonna be getting rid of most of them for better ones anyway?
For a base gun I'd recommend a P&J (A&K) from TaiwanGun, if you have UKARA, since it's a Polish retailer.
Over here you could be looking at £250+ for a DMR (for example G&G SR25s are around that price) but it's nothing more than your standard AEG internally.
The former on the other hand I believe are around £150 and full metal and if you are ordering from them might as well take advantage of cheap magazines and BBs and add some to your order.