Lots of good information here and statements.
My simple input will be go for consistency and reliability. Keep your parts to a couple of brands to avoid any mismatching issues.
You dont need to have the longest barrel you can get, find the one that is the most comfortable and make sure you match your cylinder and hopup to the set up you make.
Choosing an M14 is good for this set up, and you will match the part. A DMR can be any rifle in airsoft honestly, but a DMR impression means having the right rifle to look the part, as alot of airsofters do care about
I myself play a DMR role, but my rifle and set up is in no way that of a real DMR. My point is, if you are ready with a good budget to go the proper distance with an M14, go for it but be ready to shell out more than if you just properly upgraded an M4 with the right barrel and compression system.
Its worth it when done right, Manchovie and Pestobanana up there can testify to that, but they can also show you the tear stains on their wallets from it.
In the end, aim for speed of bb to target and consistent shots and you will actually be effective as a DMR player. If you cant really meet these then you are just another Assaulter who hangs back.