I'm looking ahead to older age, so location is top of the list. I wouldn't look at anything not near to shops, restaurants and bars, GP, library, public transport and so on. So basically in a town centre.
I wouldn't buy on an estate.
I'm not keen on en suites - maybe one in a guest room, or if I lived alone, but I don't want to share a bedroom with a loo in it.
No open plan. Maybe a kitchen diner at a push (although I prefer a dining room, so it can be another sitting room when we aren't eating), but definitely a separate sitting room.
Agreed about stairs in the reception rooms. I like a hall.
No downstairs loo would be a probable dealbreaker.
I don't care whether it's terrace/semi/detached. I've lived in terraces that are much quieter than modern detached houses with attached gardens, but I'd want one with a back lane that bin men can access, so there's no need to drag a wheelie bin through the house or have it at the front.
Some sort of parking space is necessary. Preferably a garage - not so much for a car, but for husband's Very Important Things that he never uses but would have to be stored in the house otherwise.
A massive garden would be scary. I want enough to keep us off the road, so nobody can look in the windows, but I'm not looking for gardening maintenance.