A couple of items that are generally an issue with the 310s of the year you are looking at can be the damper plate and the compression post.
The damper plates that Catalina used are not good quality and will go when the engine is idled too low. This engine should be idled between 1,000 to 1,200 RPMs. Motor mounts could also be an issue with too much low idling. Oh, and check the bulkhead behind the exhaust elbow. It's too close and didn't initially have any insulation. Some caught on fire.
On the compression post, some of the first models had a post that was too short. If this is the case, Catalina will send you an extension (about 1/4 of an inch) that you can install without too much work.
I would bet you will find a fair amount of differed maintenance.
If it's this boat on yatchworld.com (
http://www.yachtworld.com/boats/2000/Catalina-310-2444733/Annapolis/MD/United-States) you can see in the photos that the chain plates and/or stanchions already leaked. Not too uncommon, mine had the same issue. Some people that buy new boats don't do the maintenance they should and it happens. Make sure the surveyor uses a moisture meter and finds how wet the decks are around any penetrations. Mine only went a couple of inches, so rebidding was all that was required.
This boat is listed as having a dripless stuffing box. If that was installed at commissioning or close to it you might have another costly project. Those are generally have a 10 year life to them (at least official, many have lasted longer). That would require removing the prop and the shaft. You might have to replace the transmission coupling. It's not too bad of a project if you do it yourself but could cost a lot if you pay a yard to do it.
Oh, the Flyback chargers that come stock are kind of crap. If it still has that you would want to put that on the short list for replacing. The massive 4D batteries might also be ready and do you back a favor and look at going with smaller batteries. Either some group 31s or 4 golf cart/6 volt batteries.
The owners group for the C310 is really good. There are a lot of knowledgeable owners and someone has probably already dealt with any problem that you may have.
Some specs will list the shoal draft at 4'4", it's really 4'10". Not a huge deal for most but could be a problem if you need those 6" to get into your home port.
The one I put the link up for seems to be a little under priced. So if that is the one you are looking at you might want to find out why. From reading the listing my guess would be the lack of radar and chart plotter and the deferred maintenance. Still could be a really good boat.
From a general C310 perspective, we love ours. We have looked at many bigger boats including the C34, C36 and C38 and find that the C310 is much more livable for the space. We are getting ours ready to sail down the east coast and through the Bahamas and the Caribbean. We have had ours in some pretty gnarly conditions and find she is capable of handling more then we are. We have done a fair amount of modifications to ours and you can read about most on our blog. I also highly recommend you read every post on the C310 forum. Lots of good info out there.
Good luck and fair winds,
Jesse