Not all rabbits will be affected with calcium build up problems, but for rabbits that are prone to it, it's important to keep on top of it to try and prevent the build up as best you can. For some rabbits it's just minor and can be managed fairly well, and for other rabbits it will take more intervention by you and your vet to keep it managed.
If you want good pet brands of hay that you can just order and usually not have to worry about the quality being good, I've seen members on here recommend small pet select and rabbit hole. Oxbow also usually has decent quality hay. Buying bulk amounts will save you more over buying small bags.
But the cheapest way to buy hay is from a farm store, farmer, or possibly a horse barn. I've always bought mine from a farm store or farmer. Buying a bale, you do have to make sure you're getting good horse quality grass hay(no mold, no noxious weeds), that's never been wet, wasn't baled wet, has a decent green color to it and isn't sun bleached inside the bale(some sun bleaching on the outside is usually ok), isn't full of weeds, isn't too dusty, and the hay isn't crumbly.
If you don't feel comfortable knowing what to look out for and making sure the hay is good, you might be better off going with one of the reliable pet brands for now. Second cut timothy is usually a good cut for rabbits. Orchard grass is also a good grass hay for rabbits and one rabbits usually like, and may be a good hay for preventing tooth overgrowth. Rabbits usually really like oat hay too, but if you get this hay, you do have to make sure there aren't many mature seed heads in it as that would add too many carbs to the diet.
If you want to make sure your bun is going to like the hay before buying in bulk, maybe order a small amount to start and if it's a good batch that your bun likes, order a larger amount of the same batch. All hay crops are different, and apparently it can really matter to some pickier buns. I've had my rabbits turn their noses up at a new bale that I thought looked really nice
I thought I'd also add a few cautions about interactions with cats and rabbits, even though he's used to living with cats. You just have to make sure that if he does have any interaction with your cats, that no biting or scratching happens. Even just a minor scratch from a cat, can end up being deadly to a rabbbit. Also you don't want your rabbit having any access to the cats litter box, as toxoplasmosis can be passed to rabbits from the feces. And clay and clumping cat litters can cause serious health issues for rabbits if ingested.
https://wabbitwiki.com/wiki/Relationships_with_rabbits
I would have baby gas drops(simethicone) on hand, as an upset stomach from gas pain is one of the more common issues to occur with rabbits. I also like to have a recovery food mix(oxbow or sherwood) on hand in case a rabbit needs syringe feeding(upon instruction from your vet). Meloxicam suspension(NSAID) is also good to have on hand, but that would require your vet prescribing it.
https://rabbitsindoors.weebly.com/first-aid-kit.html
https://rabbit.org/gastrointestinal-stasis-the-silent-killer/
Free roaming a bunny, you do need to make sure to bunny proof pretty well. A more destructive rabbit will need more bunny proofing. But wires always need to be protected. You've got a jump on that, so that's good. Make sure to keep your charging cords out of reach, as a bun can snip through those before you even have a chance to react.
https://rabbitsindoors.weebly.com/bunny-proofing.html
https://wabbitwiki.com/wiki/Bunny-proofing
