Couldn't agree more with Melanabb. Admittedly, you do pay quite a bit more buying from a franchised dealership, but you should hopefully get much more of a guarantee in so far as they will hopefully have checked the car fully, diagnostics, everything. Many independent traders won't have access to dealership diagnostic equipment and even they sometimes have to take a car to a main dealership for diagnosis, if they can be bothered! Let's face it, the independents are selling Bertie Bassetts, they can't possibly fix and diagnose every make of car they push through themselves. That being said, there are still some very very good independent garages with excellent reputations, so don't be put off going to another with sound testimonials.
Mushsgirl, you have had a very unfortunate experience buying the 500 you had, but you at least had the foresight to pay for it, or part of it on a credit card and that at least gave you plenty of financial protection. Make sure you do the same for the next purchase, whether that be from a dealership or independent. You can of course buy privately, but that can be an absolute minefield, traders posing as private sellers, private sellers who don't declare accident damage. I'm not saying they're all bad lol! but naturally, if folk think they can get away without declaring something, they bloody well will!
You'll be struggling to find anything really good under £5.5k with low miles, low tax, insurance etc. I know the 500 is a really good looking little car, but unless you can inch yourself toward a Fiat dealership and pay the premium that comes with it, you'll be risking it and it sounds like you definitely don't need the hassle!
Best of luck!