Well traditionally the biggest problems are the CDI (if it is an early cx) and the stator. Easy fixes though. The cam chain and mechanical seal are also commonly replaced parts. but once replaced they should last a good long time. And you will get more then enough help from anyone here on fixing your possible bike up.
If you can tell us a little about the bike. IE year millage is it a standard deluxe or custom. Does it run, turn over, electrical work etc. and what does the guy want for it.
Also worth mentioning is your location. Some areas see higher prices. Worth $1000.00 in one place may be worth $400.00 to $500.00 where there is more of them to be had. Check Craigs or Kijiji in your area and remember these are usually asking prices. Showing up with a handfull of cash can quickly have the effect of lowering the asking price. Let the guy you came with to help load it on the truck hold $100.00 or so in case you have to "borrow" it from him to clinch the deal.
If its dry were it should be dry, and its wet were it should be wet, if its supposed to turn and it turns, and not move were its not supposed to move, then you should be OK.
Seriously though, they are pretty simple to work on and a lot of fun.
We could tell you a list of problems but if you have any mechanical ability and basic tools I can say that you will be able to do 99% of the repairs just fine. And it will only cost you parts and time. One word from the wise is if you buy a more expensive complete bike without problems you may be ahead in the long run if you choose right.
Many have bought $1000 bikes and had to do little to know work on them and others have bought $300-500 bikes and invested $500-700 to get things working perfect again.