Lots of bollocks in this thread.
I'd say an M4 base is ideal, there are a lot of after market hop units that fit, the Fusion Engine can be installed with minimal effort and then you can make it look how you want as well as perform how you want.
The Fusion Engine's strongest suit is that it's consistent to within 1fps and doesn't suffer a trigger response hit when you ramp the power up.
Frankly, if you're blowing the best part of £700 on a Polarstar set up and you aren't making a DMR out of it, then you're just wasting the money. An AEG can cope with all other build types just as well, if you're looking for accuracy and range then P* blows everything non-HPA out of the water.
HPA is all about the shot to shot consistency, if you're buying one to spam it like a pleb then you're doing it wrong.
A mate of mine built a P* DMR using an AR base, he built the gun around the Fusion Engine rather than buying a base gun though.
The thing's a f*cking beast, puts spring snipers to utter shame.
He's got some videos on YouTube, channel's called TheManCave I think. I'd link you but I'm on my phone.
I would personally advise against an SR25 base, because they're a 7.62 platform they traditionally have either an extended V2 gearbox and a standard hop, or a standard gearbox and an extended hop.
You can only get standard length V2 versions of the Fusion Engine, so you have to make up the rest of the space with the hop unit.
Either by getting a dedicated long hop unit - the G&G SR25 one is the only one I'm aware of - or by modding a standard hop to fit the larger space.
The former massively limits your hop unit options, whilst the latter opens you up to a world of arse pain with regard to making your modded unit properly air tight.
So just using an M4 base, where you can fit the Fusion Engine and choose from a massive range of high end, after market hop units, makes it all a lot easier.