Base guns don't have to be expensive, it depends on what you're looking for. G&G and CYMA M14s are inexpensive and can both be easily upgraded. G&P M14s are a little more expensive but they look & feel incredibly good.
It's the same story with M16 length rifles. G&G make some really good ones.
It's not always the case of spending £££ on something like an A&K or G&G SR25. Any airsoft gun with a minimum inner barrel length of 363mm (standard M4 length) has potential to be upgraded to DMR standard.
Again, the base gun is entirely down to personal choice, however choosing something conventional like an M16 will lead to more compatibility with internal upgrades and external parts. Ancillaries like magazines, mag pouches etc are also very easy to get hold of for M16s and other 5.56mm based airsoft weapons.
Choosing an M16 would also free up most of a budget for upgrades.
Of course, you can't just buy an airsoft DMR style weapon such as a G&G SR25 and expect it to perform like an M14/16 someone's upgraded to DMR standard. Creating a DMR from new is time consuming and requires a lot of effort and £££. You do get the satisfaction of having something you've built working exactly the way you've intended.
If you don't fancy spending loads of time and money converting an AEG to DMR standard yourself, you can always pay a tech to do it or buy second hand.
As for the upgrades, here's a list of what's in mine:
Prometheus tightbore barrel w/R-Hop
Super Shooter Gears
Nano Gate AB Mosfet
SHS Hi-Torque Motor
Lonex Compression Parts
Air-Lab Sorbo Pad to correct AOE
Prometheus M120 Spring
This lot's in my VFC M27. It's also locked to semi via the mosfet switch, which is vital when building a DMR- it only counts as an AEG if it's not locked to semi, and violates site AEG FPS limits.
Using LiPo batteries is also a very good idea. The trigger response of a LiPo vs an NiMh hooked up to my M27 is extremely noticeable.
I've got my M27 doing 410-425 FPS on 0.20g BBs.
If you're serious about creating your own DMR, it might be an idea to visit this thread. Airsoft Sniper Forum are the generally agreed non-official authority on creating long range airsoft guns.