How to assess deal with termite damage
If you want detailed evaluation and estimate, get a contactor in and (probably) pay for that analysis. In this situation you want them to tell you what the worst case is likely to be, so you can decide if you are still interested and for use as a negotiation tool if you are. ("It's going to cost at least $XXXX to fix this, possibly $XXXXX. Unless you knock that much off the price, the deal is off.")

Personally, this sounds like more trouble than I would invest further in, and I'd cite the termite report and walk away now, unless the price more than makes up for this. 

Be careful what you commit yourself to.
I'm reminded of a neighbor's house which they bought as a fixer-upper, only to discover that it was off the foundation and had a cracked main beam. Nobody was willing to touch either for fear that the other would let go. In the end their least expensive option was to demolish and build from scratch.