Depends on whether the conversion altered the front of the house - as in, did they turn what used to be a garage door into a wall with a window in it? If so, then yes, planning should have been sought as it is a change to the front elevation of the house.
If planning was not required (only if the front elevation wasn't altered) then at minimum you should look for a cert. of compliance with the building regulations - if the vendor doesn't have one, try and get your solicitor to ask them to get one before you sign. (a common problem with garage conversions is that the head-height doesn't exist to call it a bedroom - similar to an attic conversion, this would be noted on a compliance cert.).
Odds are everything is fine with the conversion, however the same issues will crop up when you try and sell the house again, so you may as well push to get them sorted now and at this vendor's expense rather than yours.