Hey, I feel building a polarstar DMR would be a bit of a waste of money, but I am all for a HPA setup. There is a polish site called mancraft offering a semi auto HPA system for about £160. From what I have seen this is as consitant as a polarstar (amped airsoft video on youtube I think), just with the right regulator and is dead silent too.
If you are looking for a DMR base then I would definatly recommend the A&K SR25 - a great body to build from. A tip would be to look at some polish sites to find it as last time I saw you could get it for £160. Combined with a scope, and harris type bipod you have the externals done. Another plus is the rails as they allow you to lock down the outer barrel. It is the gun used in this thread: http://www.airsoftsniperforum.com/43-longrange-aegs/892-holy-grail-building-your-dmr-aeg.html
Once you have got the A&K SR25 and installed the mancraft engine, you only have the hop system left to go. Firstly, you don't need to spend a fortune for a great hop unit, I would recomend the lonex. Seondly the bucking, again I would go for the lonex 70d bucking as in my opinion it does the job better than any other bucking.
Now this is were it gets complicated, you could go full out with ER hop's, LBR mod and PDI barrels, but in the most part these are for those who are trying to squeeze every liitle bit of peformance out of there DMR and are very difficult to install, however the peformance is phenominal (e.g. headshots at 300ft +). You proberbly don't want to go down this road.
What I would reccomend is a Promithius 6.20 inner barrel, it uses an extend flat hop hop system and an wide bore, it is also relitivly cost effective (don't use the bucking it comes with, just flat hop the lonex). A HSA barrel stabilising kit, very easy to install drop in upgrade and EM nub for the flat hop.
This would result in great peformance, with only very easy modifications needed and the build is quite cheap, this is an estimated build total:
Mancraft HPA system £160
A&K SR25 £150
Lonex chamber and bucking £20
Promithius 6.20 barrel £35
HSA EM nub and BSK £20
Nuprol 0.43g BB's - watch out you will get insane joule creep.
Total: £400 - The price of a polarstar engine.
Of course you need a reg and tank - for this I recomend a ninja SPL tank and a ninja regulator (import these from america)
Also if you wanted to go down the route which involves heavy modding, contact me as I can do all of it.
Hope this helps,
Charlie.