[willy]

You need to give us more info, a pic or two would help. How soft are these soft spots and where. If you are talking about the removable deck covers for live well and other storage they are a fairly easy repair. If you are talking about a lot of flex in other floor areas the work is extensive but can be done, several here have done it.
Major concern would be if the transom also is soft, do a search on transom on this site and get the scoop on checking it out and do so.
If it needs a transom and floor work and you intend to fish in it anytime soon you better be prepared to work for weeks to get it done.
Me personaly I would get more details and check out the boat for specific problems, if it has a bad floors and a bad transom I would look elsewhere unless you want to get a project boat and that is your desire.
If the trailer is good and the engine is good you could low ball the fellow and sell them separatly and make a few bucks as long as you can part out some of the boat and don't have to pay to get rid of it. JUst a thought.