I am looking at getting a Ducati Monster that is 400cc and 1996 but has done around 42,000kms. How many kms is to many before you will start to run into problems
According to the Ducati Monster 400 owner's manual there are several major things that should be replaced every 20000 kms meaning the bike your looking at should have had these things replaced twice already. Chances are that if they are selling it they didn't bother with the latest expensive maintenance items. You'll have to inquire about those things or they'll be the problems you'll run into.
Looking at the owners manual it looks like the major problems you'll run into at about 40000 kms if the maintenance isn't up to date are:
Those are some of the things that can go wrong. As long as the bike you're looking to purchase has had these services, and the other more minor services mentioned in the manual, performed in the last few thousand kms then it could reasonably continue to be problem free for quite some time. (intentionally vague)
I hope this information helps.
Pictured below are the applicable pages of the owners manual.