When / how was it descaled?
Look out for internal leaks - look for green or white marks around fittings and boiler, or water marks in the bottom of the inside.
Try the water and steam and see how it looks. Empty the boiler (as much as you can) into a cup (being careful how you do this so you dont damage the machine) and let it cool. Smell it and taste it. Maybe even swish the machine around first and then do this.
Look for evidence of a water filter (where is the machine? Most big cities have very hard water - FNQ and NT are better). If filtered water hasnt been used, then its probably in a scaly state. Do Gaggias have a pressure gauge? If so, see how quickly it moves from cold - hot or back.
Look up part costs online and be prepared to get bits fixed. Are you happy with doing that yourself? If not, it will cost you more.
Hopefully a Gaggia owner will give you specifics to look for :) I have no idea of the cost.