The first question is: what is their skill set. To successfully do up a house on a budget the owners have to do a lot of it themselves. If they are not prepared to tackle work like fitting kitchens and bathrooms, doing plumbing, plastering and carpentry, then forget it. The second thing to do is get a through survey, if they do not have the knowledge themselves.
We are serial house renovators and the key to it all has been that DH is one of those people who understands buildings, he is a marine surveyor, and such skills are transferable and he can turn his hand to anything. He has rewired two houses , one to French regulations, one to British and had them inspected and passed by professionals with out alteration, He has fitted at least 6 kitchens, done bathrooms, knocked walls down, built walls etc etc. I am totally cack handed so I am the navvy and decorator, garden clearer and replanter.
Match your project to your skills, There are a lot of perfectly decent houses for sale that have been lived in for 30-40 years by the same person, probably an executor's sale, where there is little wrong with the house structurally, roof and walls are good electric wiring fine, even central heating OK but the decor is tired and old-fashioned and the kitchen and bathroom need a refit and the garden needs weeding. Houses like this can be lived in quite satisfactorily while the house is gradually renovated as time and money permit and are good starter projects.
I suspect that this is the kind of property your DD and SiL should be looking for Grammaretto
Buildings-at-risk, are definitely for the experienced. They are usually Listed Buildings, with all the complications of complying with Conservation rules etc. They will also be very difficult to mortgage. A decent standard tired looking house should be mortgagable. A Building Society on occasion will put conditions on it. We once had to rewire the house within 6 months and have a woodwork survey, but DS bought a flat in a modern well maintained block, where the internal condition was dreadful (think elderly man sitting in it for 10 years chain smoking and never opening a window, or doing much cleaning) with our mortgage problems. It took three weeks to scrub, clean and paint, a new kitchen and bathroom were fitted in months plus new storage radiators. The value increased by 50%. Total cost in current terms, comfortably under £10,000