'Allo!
Kudos rescuing - brilliant move. You'll normally be expected to give a donation if you are rehoming from a charity. Anything else is dependent on where you are rehoming from. Check with owner/fosterer about vaccines (when they were last done), whether he or she has been neutered (big big thing, especially if you have cats around the area) and micro-chipped.
Yes, it does depend on the cat in regards to outside/inside debate. Our kitty is an indoor one because we live by a main road, but she far prefers it; she has very little interest in outside other than staring out the window. If you do have him/her indoors, make sure there is plenty of places they can hide/sleep and that if you have lots of holes or open spaces, you barricade the ones that could cause harm. This sounds difficult but it isn't too bad - we are renovating our home so believe me, if we can do it, it can be done!
I'd always have a litter tray indoors, even if they are an outside cat. Bad weather, illness or plain apathy to go exploring may mean your cat needs to be indoors for a stretch of time and it's best to have one in that instance. I'd get a fairly big one; our cat is average size and she dwarfed her first tray!
Make sure her/his food is away from his litter tray though. It's tempting to put it all in the same area but don't - cat poop smells!
A scratching post, bed, bowls and toys are pretty good. To be honest, we bought our kitty lots of things and she turned her nose up at some. A lot of cats just prefer boxes and fleeces.
Pet insurance doesn't need to be much; if your cat is older or has health problems you can run into some issues but for a 4 year old cat, ours is about £11 a month and caters for pretty much everything you could imagine - we took out a premier package because she's a British Shorthair and they can be prone to respiratory issues.
Agree with others as to worming/fleaing. Booster injections once a year too. Some people say indoor cats are less reliant on these things but I'd do it nevertheless.
Our kitty has a combination of wet and dry. She was eating just dry when she came to us but was pretty uninterested in it and it was the heatwave period so I worried about her getting enough water as she was a bit reluctant to drink (she was scared of her shadow). Depends on what your kitty has been used to - generally speaking you're supposed to carry on with what he or she has had and phase it out. Food doesn't need to be expensive - cats who do suffer from gut conditions may require special diets though.
Some cats will scratch. Ours doesn't much, but our house has lots of good scratching places that isn't ruining anything (think timber etc). You can get spray to deter cats from scratching though.
You'll probably never be able to use the toilet or bathroom again without your kitty coming to investigate. It's a morning ritual of ours to go to the toilet within five minutes of each other now, after having our morning greeting.
Cats are very, very independent. They don't need constant fuss and they don't need constant occupation. They will tell you when they're bored of you. Because of this they can be left for long periods of time, although they need feeding obviously. It's lovely to come home from work and have her meow and purr at you, and we adore her because she's very affectionate in her own way. Not all cats will want to lay on laps or be stroked for hours on end and some, perhaps more likely if they've had a bad home, will be nervous and scatty. If you are rehoming from a centre, they will ask questions about what kind of cat best fits your lifestyle and needs.
Hope that helps 