English driving licence in Spain

As you will not be resident and subject to a maximum 90 day stay, you will be fine driving on UK licences. (You would not in fact be able to get a Spanish licence)

As to where to buy … well, cars hold their value in Spain far more so than UK; if you are happy with a RHD car on the right hand side of the road, it would be cheaper to buy in UK … THAT BEING SAID … you would then have to matriculate to Spanish plates which can in some instances be costly, plus you’d be liable for 31% import duty and tax on the declared value of the vehicle … so if all you are wanting is a cheap car, then on balance it may be better to get one in Spain.

Be aware that it will of course require ITV every year

EDIT:  just thinking on this - you may run into issues both ways- 

If you buy in Spain, you’ll need a NIE (which can be obtained by anyone, resident or no) and to register to your address you’ll need a Padrón - I am given to understand that non-residents now may NOT be included on the Padrón - but I can’t say I am 100% certain of that.

Buying a car in UK, no issues - but matriculating to Spanish plates … Padrón required again….

I would recommend getting some solid advice from a solicitor here before you commit - but make sure it’s a solicitor that specializes in migration law.  I’ve heard too many stories of people who used the solicitors that they used to buy a house and were … less than satisfied … with the results.