I think I have mentioned it on the site before but I used to specialize in buying termite houses. In Arizona they are quite common, in the Midwest not so much.
I disagree with Keshlam in saying it is too much trouble, and walk away. I think the sensible thing to do is figure out what you want out of the house and what it will cost to fix it.
If you want to buy a house an just move in and live there and touch it very little then you shouldn't buy a house with termite damage. In actuality your house choices are going to be extremely limited.
If you would like to update a house and make small living changes, this could be the perfect house for you. In the flipping business we know it is much more cost effective to gut something then work around everything.
How do you assess termite damage. You do it yourself. That is fine that you had someone help. I would go around the house with a fine tooth comb and document any kind of wood damage, any kind of paint damage, any sagging or structural issues. Everything should be attributed to termites.
So the main assessment is centered around if the termite damage is on the outside of the house - most houses are like this or is it pervasive through the whole house. You really need to spend a good deal of time in the attic (trusses, floor beams, and plywood sheathing under roofing) and in the basement looking at the structure. Framing and cosmetic things on the outside of the house can be fixed easily.
The next step is to get a quote from someone that can come in and fix everything on the outside, drywall and simple framing. Double that quote and take that off the price of the house. Remember that home owner has to now disclose this damage to all new people who look at the house. Mention this to them and use it as a negotiation technique.
Really if you aren't looking to make money off the house you need to figure out if you want to manage the fixing of all this stuff to make it look exactly how you want or do you want something that is more move in ready. There is NOTHING wrong with buying a house with heavy termite damage, you just need to set your expectation and it would probably be cheaper and faster if you didn't live in the house while the inside stuff was getting done.
Note: I mention to double your quote because I have found from experience that most termite quotes end up on average at 1.5 times original quote. Double it because you are managing it and might not be living in house for 1-3 months depending on damage. Also I should point out, be fully aware of any asbestos potential (think flooring, popcorn ceiling and ductwork). This is an important caveat because when you start opening stuff to fix termite damage this is really the only thing that can prove to be costly real quick.