First of all, I would say you need to set a budget. You need to be honest with yourself - what can I afford - and this may involve sitting out of a few gamedays to get your gear - or selling up something from another hobby - but the crux of it is, to get a good start on the gun front - I'd say £200 is a bare mimimum you should consider acceptable.
Now that you've got a budget you need to decide if you want to head in a specific direction with what you buy "I wanna look like US Special Forces" - leads you down the route of an M4. "I wanna look like a Royal Marine" would be an L85A2. "I quite like the idea of doing a spetnaz look" would have you perusing AK's and finally "I dunno, I just want something cool!"
I'm going to help out person number 1 and 4, and ignore the pleas of 2 and 3 on this one and centre myself round the M4 Carbine. Why? First of all, they are the most commonly available design of Airsoft Rifle. This generally means they are the easiest to get parts for, the easiest to upgrade and the easiest to seek help with (a bit like owning a Ford Escort instead of a Ferrari F40 - more people will have had an Escort and thus may be able to help you out!)
The next question you need to ask yourself is what type of player do I want to be - as you need to decide whether to go down the route of using High Capacity Magazines, or Mid Capacity. The Pro's and Con's of these will be fleshed out seperately but for now, we'll assume you go down the high capacity route to start with - as its easier and doesn't force you to run out and buy a chest rig / plate carrier straight away.
You may now be tempted to say "So we've decided on a M4, and high cap mags with a £200 budget - lets do a search for £200 AEGs then!" but I say hold your horses - there are other considerations to make as well.
First of those is batteries and chargers - the next is magazines. I'd say if you are running an M4 on high caps (generally 300-450 rounders) you are going to want at least two.
My advice is that if the mah rating of the batteries you are looking at are under 3000mah - you're going to need two to start with (as no batteries = your not playing.) You'll need to consider what type of M4 you buy before you buy batteries for it - but you'll also need to think about a charger too - but that comes after the batteries. So back to the M4 for now.
If the budget is £200 I would say no more than £150 can go down on the rifle - and I would only spend that much if it comes with at least one mag and at least one battery - if it doesn't drop the budget accordingly.
The nost important message here is BRAND. Buy a Branded Gun. The higher end stuff includes TM, G&P and G&G Top Tech but more in this budget territory you will find G&G Combat Machine, JG, CYMA and others. If it doesn't have a brand - check on the forums - get advice - search google for reviews (not written by the retailer!) and get a feel for what you are looking at. If the brand is SHITEO-3000 chances are the box will be worth more than its contents.
It's out of stock at the time I write this but when they get them back in Pro Airsoft Supplies carry the G&G CM16 Carbine at just £99.99 - no batteries, no charger, but 1x 450 round high cap comes with it. Its front wired and the batteries go inside the front headshield so it will take a mini sized battery. A quick nip over to Component Shop gets us two 9.6v 1600mah Nimh Mini Batteries with the correct connectors for £30 posted. Add another £15 and a "smart" charger can be added and that means you've spent roughly £150 (allowing for postage on the rifle) leaving you a massive £40-50 of budget to play with. For that i'd invest in some magazines and would consider a nice optic or some other accessory from Ebairsoft or the likes - you'll wait a week or two for it to arrive but its much cheaper than buying from within the UK.
This is, of course, just an example of what you can do with a small budget. If your strategy is more defined in terms of where you want to end up - you may want to play it differently - we are all individuals but this is just an example of one way you can review the situation. The critical message is to consider the full package. Spending all your cash on an AEG to find it comes with 1 midcap and no battery or charger isn't a big deal to a long-term airsofter with a bulging collection of magazines and batteries - but if you are just starting out it will strand you with something you can't skirmish - and leave you with no cash to sort the situation or even go and hire for another game!
Second hand is another option - but with that I will urge caution. The airsoft community is full of well intentioned, courteous people but unfortunately like all things in all walks of life - there are some a-holes as well - and you can find this out the hard way. I would always consider buying new when first starting out as you want something that is going to be reliable as its your first weapon - but not only that you want to have something with the reassurance of a guarantee in the background - should the worst happen.
Basically, its a mindfield and no there is no such thing as too much soul searching (about what you want) and research (of retailers, guns, recommendations) that you can do. Never rush the decision and make sure you've covered off all the basics. The gun, the mags for it, the batteries for it, and the means to charge them.
As a site operator I've seen too many times people get one or more of the above wrong - and have gone home unhappy or had their day cut short because they don't have everything they need.
NB: the retailers above have been quoted for "real life reference" and this DOES NOT constitute a recommendation or referral! Do your own research! Oh, and one last nugget from the forums - if a retailer has the words "BB" and "guns" in the title - as a general rule - don't touch them with a barge pole! The sturdiest thing you'll get from them is the box they posted the kit to you in!