As others have said, it might be worth just going the normal M4 route, as you'll have a plethora of customisation options. If you're dead set on the M14 look then i'd still keep it as a full auto AEG. DMR FPS limits tend to fluctuate between sites as there's no "standard" so there's a risk of you not being able to field your DMR if you do decide to try different sites. I'd only start thinking about making a DMR when you already have an full auto AEG.
Also you got to take into account that a lot of sites have a minimum engagement distance for snipers/DMR's and carrying a pistol is a requirement if you do decide to use a sniper/DMR, for you to be able to engage targets who fall below their MED.