Aspen litter is fine. Your other options are Carefresh, Feline Pine, and Yesterday’s News. If you want cheaper litter, look at other stores (Feed store/Tractor Supply/Lowe’s/Home Depot) and get horse stall bedding or wood stove pellets.
I wouldn’t get any toys at the pet store. Most of what they have there is either not safe for rabbits or not enticing, therefore a waste of money. Online stores will be your best bet for things such as willow and natural wood chew toys, as well as grass balls/mats. You don’t want any of those dyed wood toys, nothing with bells or nuts inside, and definitely nothing with any type of honey or seeds.
If he’s not litter trained yet, it’s likely he may have a few accidents in the cage bottom, but that’s life. Quickest way to get him trained is to remove the bedding material and only have litter in the litterbox. It’s too confusing otherwise. You’ll just have to be on your toes and clean up any pee accidents ASAP. If he steps in pee, wipe his paw dry (if he lets you) and he’ll groom himself.
A plastic bottom cage won’t typically cause sore hocks as long as he’s allowed access to other surfaces during play time. If his litter habits improve, you might be able to provide a blanket or pet bed, but at this stage, he’d probably just pee on it.