Sarah Like Cars the condition all depends on how hard they where driven and same applies here really so the only way you'll really truly know is by speaking to the operators or the Engineers who service them or buy direct from the source owner and trust they tell you the truth about how it's been driven.
Dealers are dealers they will always bias the information they pass in there favor to make the machine more attractive.!!

Regards new or second hand then I can't help has there's so many variables and very specific to the machine and the use your putting it too and what they were doing.!!.
What I can say is I have an friend/acquaintance who runs a CNC shop that's running 24/7 both VMC and Lathes and I wouldn't buy is old machines has they are nearly ready for the scrap heap.
The problem comes from the fact they are so very expensive they need to be run constantly to cover the cost and every minute stood still cost's money. This will also apply to other CNC business's so should give a guide to the kind of life most manufacturing machines will face.

Also realise that there are other advantages to New machines other than being shiny and reliable.? . . . Often they are more efficient so cost less to run and increase Productivity. The same friend recently bought a new Index 5 axis lathe that is producing parts at 3 times the rate of is nearist newist machine that is less than 5yrs old and cost's less to run per hour.

Have you looked at leasing.?