I wouldn’t be expecting perfect litter habits before a spay, but I would definitely (and currently am with two babies) start litter training as soon as possible. My 16 week old rabbit is pretty much perfect right now, though he does have the odd poop outside of the box. My 11 week old is pretty much hopeless at this point, he’s finally starting to get the idea and pees in his box about 80% of the time but the poop will be an ongoing issue until he’s fixed I think. So you can have some success even early on, but it can be a struggle! Just make sure to have some vinegar handy and wipe up as much pee as possible so that she doesn’t associate the areas she’s gone already as prime bathroom areas.
As for the bunny castle, I would wait on that one. You can get boxes pretty cheaply (or free) pretty much anywhere. I often go to Costco and grab a few boxes (I usually avoid the ones that carried non-packaged produce or with any mystery stains). I honestly don’t think I’d ever buy a castle/maze for them just because of how easy it is to find thick sturdy boxes for free (although they certainly don’t look as nice!).