We managed fine with 2 litter trays OP, although we scooped as soon as our noses told us it had been used. More important than the quantity was the location IME; we had one on each floor once the kittens got the run of the house after we quickly discovered that (much like small human DCs) the kittens didn't like going downstairs alone in the middle of the night for a wee. Maybe start with 2 and see how you get on? They are cheap and readily available if you need another.
We've never had a pet plan. I suspect whether it's worth it will depend on how lucky you are with their health. For our senior cat, it wouldn't have been worth it. She came chipped, neutered and vaccinated, goes for her annual vaccination, and has only ever had fleas once in over 15 years despite spending many hours outside and only being treated for fleas and worms 3-4 times a year. We wouldn't have even benefitted from the savings on other treatments as up to 2 weeks ago her only other visit in her life was for a skin issue. This last fortnight she's needed a dental and has started monthly arthritis injections. The former wasn't covered by the insurance, but the latter is so I'm not sure even this would have been worth 15yrs of paying for a plan.
We use price comparison sites for insurance. I would be tempted to take out full cover for the first few years as they are most likely to get themselves into a scrape as a youngster and it gives time for any health issues to show. After that (if possible) it may be cheaper in the long run to have accident-only cover and put a sum into a savings account each month instead to cover the inevitable old age care. DCat1's insurance is now £100pm despite never claiming until now, and I expect it will shoot up more now she's used it. The amount we've spent in 15yrs is unlikely to equally anything near what we claim.
I wouldn't bother with cat beds. Buy a nice snuggly blanket for the sofa (layed out in a manner that indicates it's for human use to increase its desirability), and pop another blanket into a cardboard box (cut a door in the side and seal the top back up) for the first couple of weeks when they might be a bit nervous.
Unless money really isn't a concern and you're happy to drop £££ on new stuff that may be rejected, consider buying a second-hand cat tree. Some love them (our younger 2), some (senior cat) will look upon them with the same horror, disgust and judgement my DM would have shown for them 😂 If you've only spent £25 it isn't a massive loss if it's rejected, and equally if it's loved, you can replace it for the fancy new version (at which point your kittens will decide they don't actually like cat trees after all... 😬).
We went with water bowls, however the senior cat will only drink out of the pond, puddles, or water that's pooled in various garden pots/buckets so we didn't even have a bowl out for many years once we realised it was forever untouched. The younger 2 will only drink from the above plus full human glasses and bowls. They don't like shallow water bowls. If the water falls more than an inch below the top of the bowl it will be ignored. Ditto if the water is freshly poured and hasn't sat there getting lovely and stale for a few days.