What would a 3 1/2 year old like? Well, it depends on what she already has, really.
At that age I was getting Ds1 a toy cooker, with a sound and light hob (which I love, one ring makes boiling noises, the other makes frying noises when you put a pan on them) and plastic food to go with it. He was at the age where he loved to copy whatever I was doing. He also got a toy Henry that has a bit of suck (but not much, just enough to vacuum up the polystyrene balls out of the beanbag). I think he also got a teaset, with a drying rack and a washing up bowl, and some toy food, both in tins/packets and out. Books, always. Toy animals, always (we have a rule - he can only have new animals that he doesn't already have).
How much to spend? That is entirely up to you. YOUR choice. As it is for everyone else.
This year I have spent rather more than I should, because I bought a playhouse for both boys - and because I couldn't find a decent size one at a cheap rate (I'm in Australia, everything is more fecking expensive here anyway) I decided to spend a bit more and get a good quality large one that will keep going for 10 years. But it's worth it for the amount of play they'll get out of it.
As a matter of interest, why is your DD unlikely to go to school? Is that a choice you've already made?