Cars always have an element of risk, the best mechanic in the world can't predict when a car is going to break down.
Just buy one if that's what you want, ignore the silly 30day warranty or whatever they may offer. Under law if a fault occurs within 6 months then by default the garage has to fix it unless the garage can prove that the fault was not present when they sold it (which is virtually impossible). After 6 months if you can show the fault existed when it was sold the the garage is still liable.
You also have a legal right to reject the car within 30 days if it is not of satisfactory quality, not fit for purpose or not as described.
Its up to you whether you buy an extended warranty on the car if it is offered, the price of the car is in my mind the governing factor as to whether it is worth it.
I'd just like to add a plug that Stoneacre is a company that at one time I wouldn't have trusted but I have completely changed my mind on, they have a number of service plans, warranties etc which seem to be good value (I'm unconnected other than a relative buying from them). Salesmen can be a pita though.