If it's already had the cambelt changed without the pump then i'd just leave it as it is until it's next due or if/when the pump starts making strange noises or leaking.
However, if it's not been done yet then i'd ask for the belt AND pump to be done prior to sale & would ordinarily do both, though i don't know of waterpump issues on the petrol Bravo's, just the 150bhp diesel engine. I wouldn't try to do what you've done with the Volvo and go several years beyond the schedule without changing it - you're unlikely to have a happy ending on a Fiat by pushing cambelt intervals THAT much!
As your'e buying from Arnold Clark i'd assume it's got a fairly lengthy warranty anyway, so if the worst does happen & the waterpump goes (which on a T-Jet is unlikely) then i'd just expect them to cough up for not changing it to begin with.