IMO secondhand 500's are way overpriced; for £5-6k you could get far better value from a different marque or model. An equivalent Panda of similar age and specification may only be half the price for what is functionally a better car; if you shop around and pay cash, £6k will get you close to the entry point for a brand new one with three years warranty.
500 residuals are driven by fashion, not technical merit. Without considerable DIY expertise, I wouldn't want to own one out of warranty, no matter how cheaply I could buy it. Many of the typical 3-4yr old forecourt cars I've seen could easily cost you another £2k in repairs in the first year of ownership, and that's if you get a good one.
Stone chips are common on all secondhand Fiats; solid red seems to be the worst colour to have in this respect.