Yeah but... Sandero... Your soul will die.
The Colour Therapy is essentially a Pop.. but with the white mirrors, antenna and wheel trims, plus the body-coloured gear knob.
If the car doesn't have pristine white (FIAT original) wheel trims, then half the reason to pay the extra premium for a Colour Therapy has gone. You'd be better off looking for a regular Pop, which are a bit cheaper.
500 is a learner car, so a lot of them probably get a fair bit of clumsy use and inexperienced owners, which is why I think they seem to have all sorts of problems. They're usually not serious and can be fixed with just minor knowledge/financial outlay.
Your one ^^ would worry me a bit, since taking a car in for an MOT, not realising the rear indicators/lights don't work.. or that the tyres are knackered suggests and owner that was better suited to owning a vacuum cleaner than a car... but it needs a look to be sure.
If the sub-frame/arms/springs at the back look rusty/crusted in dirt and the engine bay looks like it could be used for growing potatoes, it may be a "No" from me.
If it looks reasonable, check the oil level and condition (ask the owner when it was last changed, just to hear the answer) and in particular look for signs of accident damage under the bonnet. 90% of pranged 500s seem to have hit something, rather than have been hit.. so bent/flaked paint on the scuttle, odd paint shades between wings/doors/bumper/bonnet etc. might be a bad sign.
Don't go for a Panda... since you obviously want a 500 and you're only young once... the Panda is more sensible for sure.. but where's the fun in that?

Whatever you go for, the same checks/tests apply to any car. £3.5k should get you something reasonably tidy and in great condition, so you can be a bit picky, that's all.
Ralf S.