WE M4 GBB rifles use a proprietory barrel, hopup, bucking, and practically anything else internal, and of course mags.
I would 100% re-evaluate your base-gun choice for a dmr build, since you chose a GBB rifle running on un-reliable Green gas.
I don't think it would be possible converting it to semi, minuse placing a screw in the lower reciver so you couldn't turn to that select fire position.
Mags are expensive as hell, and all leak except for the brand new Gen2 mags which are open bolt, this gun is closed bolt; you will have to buy the conversion kit ($90), which will break, transitioning to RA-tech metal parts will run you $180. This gun is impractical for your wishes.
If, however you are dead set on going GBB rifle path, M4 platform, I can suggest the new KWA M4 GBBR riffles, internals are better, mags are better, and in the long run it will cost you less.
However, any GBBR is an advanced users choice of firearm, the time it takes to maintain it and understand how to fix a problem if it goes down, and it will, is rediculous.
I own a WE M4 GBBR.