Yup. Most dedicated DMR platforms will be pretty expense, and upgrades will be pricy.
If you want to do a DMR platform with anything 7.62 based have a look at G&P M14s. The latest batch have the nice non-standard body kits as stock and some come with G&P optical sights if you want to spare the expense of buying separately.
As they're M14s they have a Ver. 7 gearbox, which are supposed to be pretty easy to work on. They also have the advantage of the real steel weapon having served as a DMR for around 20 or more years in the US army, and even more in other services over there. Only problem I see practically is mag availability and expense. Pouches aren't too difficult to find- WAS do some decent 7.62 pouches and BulleUK do some as well for slightly cheaper.
If you're concerned with M14 mags though, maybe you could convert an M16 to DMR standard? external work would only include adding a full length RIS and removing front sight, as well as buying either some Vietnam style short STANAG or some short Magpul magazines. As well as this I'd look into optical systems like Elcans or the Spectre DR, as they fit in with the current sort of kit soldiers are being issued and using. Plus IMO M14s and dedicated sniper rifles look best with traditional optical sights.
As for upgrades, you'd be looking at these parts and changes:
MOSFET to allow high voltage LiPos for better trigger response
16AWG low resistance gearbox wiring
SHS 16:1 gears
A Lonex neodymium motor or branded high torque motor
Improved cylinder with better air nozzle
Firefly hop rubber with H-nub
tightbore barrel- either Prometheus or LayLax
As well as this you'd have to lock any guns with an automatic firing capability to semi automatic.
Other members may be able to help you better with upgrades, I'm fairly clueless when it comes to which brands are best for upgrades and what upgrades to actually do
One last thing; I've heard of G&G M14s having ridiculous muzzle velocities out of the box on 0.20s. If you could find one of these and stick a tightbore in it you'd be golden.