A tube amp for sure.
affordable?

Fender hot rod de luxe
Fender hot rod deVille
Fender concert II
in fact any fender TUBE guitar amp, but most of them could come expensive allthough in usa they are much cheaper as in europe.

Peavey classic 50 (the tube version!)

To be honest I don't know what SKA is?, so
if he needs more jazz sounds as pop/rock sounds the roland jazz chorus amps do very well.

I can mention a lot more but most of them are expensive...

Fred

At the moment I have;
Fender concert II
Fender Twin
Fender hot rod de luxe
Mesa mark IV
Roland JC77

The most versatile amp is the concert II which can still be found for reasonable
prices and is actually the amp I use most.