Yeah one of the first bits of advice people have is to stay away from gas as your first gun. Personally I don't think there's much that could go wrong but you do need to be prepared to maintain them a lot (much more than pistols I guess) and pay for gas which is obviously more expensive. If you take a look around on the forums you'll see a lot of people recommend the G&G Combat Machine M4s for a beginner's first gun. So you're looking at ~ £150 with a battery and charger.
Cheapest GBBRs I've seen are about £150+ although that said I don't know if the KSC Vz61 / WE SMG-8 are more classed as pistols rather than rifles. GHK G5 is £225. Then WE M4A1 ~ £270.
Also the G&G Combat Machine GBBRs are said to be bad despite the fact the electric ones are so good.
It's also said it's impossible to get a good AEG under £100, which, whilst true in the UK, isn't if you buy from somewhere such as Polish retailers, TaiwanGun or Gunfire. Providing you have a UKARA defence of course. I've recently seen a load of Cyma and JG guns on sale from 80 to 100 euros on TaiwanGun (+ £12 shipping)
Some good retailers I can think of, off the top of my head:
Pro Airsoft Supplies (Excellent customer service)
Patrol Base (very good service again)
Airsoft Zone (lots of stock and good prices)
Zero One (very good amount of stuff for sale, free skirmish voucher with gun, but two-tone paint service is bad)
AirsoftWorld (I think some people said they're bad but overall not bad store)
Redwolf UK (great, some unique products, alright pricing)
Land Warrior Airsoft (generally good, some stuff can be found cheaper elsewhere but lots of stock and things)
Fire Support (haven't used em but I know a lot of people have and no problems AFAIK)
Hope this helps and welcome to the forum