I think I will go and see it in action, any advise of what specifically I need to check will be appreciated
i dont envy you at this point... its hard to know for sure what limitations and tolorence you can live with or afford to tune out of a machine and how much time and cash that would involve.

one simple test i would do is cut a circle and see how round it is with a vernier caliper... dont expect absolute perfection, 0.1mm out would have me worried... however, it is quite likely with a nip and a tuck you could tune any problems out (if it has double ball nuts for example)

it looks to have profile rails, if this is so and they have been looked after (lube and kept reasonbly clear of machine dust) they should be fine... if you throw your weight into trying to move any of the linear bearings and you get any movement (excluding travel) they are shot (id be suprised if you find any problem with the rails but its worth a look)

get him to run the auto tool change to make sure all is well with that
(bear in mind that the tool changer will need compressed air to run)

if the spindle bearings are getting noisy they will need some effort to change