Be a little wary when buying a car with air conditioning or climate control. The phrases "need re-gassing" is basically another way of saying "it doesn't work".
It may be all that is needed is a regas, which will cost less 100 quid, but it could indicate there is a more serious fault with the system. If air conditioning is not used for long periods of time components can wear or perish as the compressed gas provides the essential lubricants.
It's quite normal for the gas for seep away over time, most vehicles need some sort of top up gas at around 5 years old.
However, it can also leak away through the various connectors, valves and other bits and bobs on the system and if it has got a leak regassing will probably make little or no difference in the medium term.
The bottom line is, that unless a seller says their air con works or blows nice and cold you should ask.
Unless you are told specifically it is working, make your bids as if the car does not have air con at all.