A proper DMR is always built up from the ground. Not saying you can't buy decent base guns but a DMR requires extreme amount of fine tuning when you really want to push it to the limit.
Internally perfect air seal, shimming, stronger motor for heavier springs, decent quality inner barrel/hop-up unit/bucking/nub, using quality heavy BB's etc...
If you can't outreach your opponent when you're forced to shoot only on semi and follow MED rules then there is no point to struggle with it.