Can't argue with what most are saying in the AR20s defence. I had one and it was a good gun after I sorted it.
However, the OP stated an intention to spend his whole 600 quid budget on a new one. I can't and didn't recommend he do that.
The thing to bear in mind for the OP is second hand values. They're a money loser exactly because of the great British tradition that says 'if it aint expensive, it's crap'. I don't subscribe to that by the way, but look at the arguments offered on this thread, the AR20 owners trying to convince non owners that they're as good as Steyr or Walther Dommi and even FTP 900s. Sorted, and in the right hands, maybe. But in the hands of a new shooter, I doubt it.
Most have stated they got theirs for 300-400 quid, if the OP can get one for that, I'd say buy it and mod it. That's because he'll get at least the cost of the gun back if he decides to sell, and he'll end up with a decent starting out gun, as he would if he bought an MPR or an S200 or 400 (which are well proved and already have excellent accuracy and quality of parts and finish)
If he buys a new one, as he stated he might, he will
lose money on it if he decides to sell later on, guaranteed!
Of course, there's always the adage of 'you get what you pay for'. Well I paid just under 400 quid for mine and spent another 400 quid making it nice, over the top I know but I did it anyway. That put me into EV2 territory on price.
Then of course, he might buy one, mod it and keep it. I don't know many new shooters who've done that though.
I suppose the ultimate question for me would be, would I have another one? Well, no I wouldn't, not even as a second gun. I'd buy and MPR in a heartbeat, or an HW100 (I've owned two), or a 400 or even a springer (except it would bollox up what little skill I have)