Having owned and operated a pest control business and being licensed for termites, I also do not see the sense in your plan, nor the how-to.
Accept that you have bought a termite heaven. If you do not remove the untreated wood, and eliminate all pathways of termite activity, you have bought a big task.
I would avoid any flip until you resolve a good plan on how you are going to cope.
Consider removing everything you can made of wood from the old structure. Old wood that must (&can) be saved can be injected with a boric acid gel that goes into a small hole and is absorbed by the wood. (Nisus=mfg) If done right it should protect well treated wood for 40 years, if it can be saved.
This would be a DIY cheap job. Trying anything to do with a trench and all the chemical gear and safety equipment and knowledge on how to do it does not make sense.
A negative attitude will come back on you.