First off, welcome to the forum.
I am going to assume that you know all the weight specs of both this fifth wheel you are looking at and your truck, and it is all good. If you don't know this then that is probably the place to start.
I would also want to what, if any modifications the owner has done to the trailer as well as consider the weight impact to unloaded weight of trailer?
Is it winterized, will you be able to test the plumbing when you go to view it?
Are the batteries in it so you can test the 12v systems?
Is power available to test the 120v systems? Maybe you will need to bring a generator to test this?
Is there propane to test the lp systems? Again, you may need to bring a full bottle for testing?
Assuming you have enough photos, can you get a general sense of condition? (did not find it on the link you gave)
How often was the trailer used? When was the wheel bearing service? Any idea of approximate miles of towing? Maybe tires need replacing too?
Will the owner allow you to hook it up and go for a short ride? Is it currently insured and licensed for the road? If not would the owner be willing to obtain a permit for that?
Hope this list helps you out some.
Good luck with your purchase.