Hello and welcome.
Have a wander through the forum; you'll find this question has been asked many times before.
As with all significant used car purchases, give serious thought to having it professionally inspected prior to purchase. There are a small minority of real lemons out there and this could save you a headache later. Most 3-4yr old cars (of whatever make) need minor repairs and replacements in their first year with a new owner; these will generally not be covered by any used car warranty. 500's are no different, so set aside a fund of about £500-£1000 to cover this.
If you tell us a little more about the car you are considering and what you want it for, we may be able to give you more specific advice.