It sounds to me like the mother of all risk profiles, and that you might have an emotional commitment after spending so much time.
If it's like most of London, houses were made with cookie cutters by speculators, either well or badly depending. For context,what were the last 10 sale prices for identical ones in various condition, after you have allowed for market movements since then?
You sound to be looking at a top to bottom normal refurb, possibly needing to go back to the brick on most of it, and potentially replacing floors, and the roof in fairly short order (<10 years), and maybe needing to do something with external render. Plus boarding ceilings etc (artex/asbestos).
That's ignoring any subsidence gotchas.
If you aren't a pro, that's the thick end of £20-£30k (more in London?) to bring it up to scratch if it *was* in decent conditon, before you do any floor/roof structural stuff or external works that you don't really know about yet.
What about double glazing (£6k?) and insulation works (external? - 8-10k more) to bring it up to a 70% EPC or so.
Plus you have a vendor who is being economical with some bits of the actualite.
I'd want to have headroom to spend £50k, maybe £60k, to bring it up to a price 10-15% below my estimate of the value afterwards, or would walk away, unless I could do some magick on the hidden risks (eg fixed price all-in renovation contract signed first), or if there was a goldmine hidden somewhere (building plot, room to add a bedroom or two at low cost).
In London I'd also want to know about Council admin buggeration during refurb (Scaffolding? Skips? Licenses for these? Closing off a parking space for a month?) and potential Licensing Costs.
Indemnity policies from the vendor may help, but all the shenanigans make me wonder if they think they saw you coming, and are trying to play you like a fish.
Refer Mad Regulators to Arkell vs Pressdram.