Hmm, I think your poor DD might be needing her own one then!
If you want to preserve their original state (and value) painting is one of the worst things you can do. It can be stripped but it's such a fiddly, painstaking job that it's really only worth doing it if you really want to or it's valuable.
The paintwork on Triangs tends to scrub up well but you'd want to be careful where the flowers are painted on.
If you don't want to cover up the roof you can good results by photographing an undamaged section and doing clever things with a printer to get a copy to patch the bad bits.
If you're keeping it for you, the furniture which would have been used with it when it was your mum's is likely to have come from three ranges - Barton's, Dol-Toi or Triang's own Spot-On range. The Spot-On is very 60's style and quite pricey but the others aren't that hard to find.
Actually, I don't think it's the best dolls house for a child because the room layout is so weird. You have that huge upper room but chunks of it aren't accessible on either side because of the way the front opens and there aren't any stairs.
Still, it's nice to think of it being loved by three generations.