If the car you are looking at has just been serviced and has 12 months MOT, then chances are you won't need to do anything else to it for a year. Reliability is generally excellent. Service interval from 2002 onwards was 10k miles or 12 months, whatever comes first. Servicing via specialists instead of main dealers can be very cost effective. I just had mine done a few weeks ago for under £150, for oil/filter, brake fluid (every two years is advised), and coolant change. I'm just about to replace the brakes on mine this week. Cost for new pads and discs all round was £325 and fitting should just take a few hours. Not bad after 4 years and 40k miles. I generally get about 12 - 18k out of a set of tyres if I rotate them every 6 months or so to even out the wear (the fronts take the most abuse).
It's worth paying a little bit more for the STi models (from 2002 onwards), as they have decent Brembo brakes, 6 speed box, slightly nicer interior and tighter handling. The WRX models are more discrete and comfier/quieter on longer journeys and commuting. My advise would be to opt for a near standard car that hasn't had tonnes of modications done to it, and keep it that way
The biggest downsides are:
Fuel economy - expect 20mpg if you like driving. 30mpg is possible on a motorway journey as long as you keep a steady speed. 10mpg is also possible
Interior Quality - not the best in the world but you can't expect supercar levels of performance for not much more than Ford prices and expect to have the same interior quality as an Audi for example. Mostly hard wearing and rattle free.
Insurance - shop around is the best advice. I'm likely to be around £400 -£420 this year for a 2007 model fully comp with protected no claims. Younger drivers with a fruitier driving record can expect to be closer to £1k.
Image - lots of older cars are now driving by the backwards baseball cap fraternity, who love their 22 inch chrome alloys, tacky body kits and stereos that could fill the Albert hall with sound. This has an impact on insurance premiums.
John
Got the Exige for when I want to scare myself and Scoob with effortless speed and power for just getting about. You lot are pish at talking me out of this. 
Then when you lobbed the Scooby idea in, about halfway down the first page of derisory posts, you'd get completed shot down and the problem would have gone away by now!!
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