DMR rant incoming.
In order to help you, I first need to know what your definition of DMR is. In airsoft, DMR can mean any combination of the following:
- high fps
- locked to semi
- externally resembling a real life DMR
What a DMR is NOT, is an AEG that can outshoot other tuned AEGs. My carbine is tuned to the point where the only thing I would change to make it into a "DMR" would be to swap the ACOG for a short dot scope, add a suppressor and maybe a bipod. Note that these are all external changes.
So since you plan on upgrading the internals, I can only assume that you want your gun to visually resemble a real life DMR. SO. Real life DMR's don't have short ass barrels and huge ass silencers. They also don't have tiny ass rails. Scratch the Raiders. The R8 is the only real option from your list, but I seriously recommend starting with a better external platform (ie METAL).
My recommendation would be to start with a gun that has the DMR-type externals you want. Something like a G&P SAM-R, G&P M4 Navy Seal, or a VFC MK12.
Also, super long barrels don't help your range or accuracy. 455mm is generally accepted as the most accurate length, but similarly high levels of accuracy can be attained with shorter barrels. Any longer is a waste of money and makes your gun unnecessarily long. I'm sure Frank is going to mention his MK18 somewhere in here.