Cats are pretty independent, that's why they work for us. Our boys are quite happy to be out most of the day, then get a fuss when we're home from work. We've been lucky because R-Cat is a tart sociable, so the neighbours know him and are happy to come in and feed them both for us.
In terms of what you need:
Cat bed - ours don't bother. We have tried various, from blankets in a box, to a top of the range fur lined thing. Nope. R-Cat is currently sleeping on a pile of clean washing on the living room sofa that I'm too much of a slattern to move or re-wash as I'm 8 weeks pregnant and quite frankly can't be bothered and T-Cat has been alternating between a roll of organza that I'm using for my table decorations for my wedding at the weekend, and the box the organza came in that I haven't moved from the middle of the floor all week, see above.
Cat flap - this was our biggest expense; we paid about £80 for the microchip cat flap, but it was definitely worth it and I'd recommend it. Only your cats can get in.
Toys - We have a few toys, but they don't bother with them. I usually only get them out when it's bad weather to distract R-Cat from clawing my sofa.
Litter tray - They haven't really used the litter tray since they started going outside, but it's worth having one for if the weather is bad. We used an old 12l storage box to stop the litter getting everywhere.
Food - I get them a high meat content, no cereal wet food, and Iams dry. The Iams is in a double bowl which is down all day, wet food in a bowl each twice a day.
The biggest ongoing expense for them is their worming/flea treatment.
Both of ours were rescue cats, and R-Cat is really sociable. When we first got him (he was between a kitten and a cat), there were young girls down our street about 5/6/7, and I'd come home to find him being carried about under his 'arms' with his 'legs' dangling, or my favourite was when they had him in a pram. Your best bet is to go to the shelter, let DD loose and pick one if you can limit yourself of the ones that doesn't run and hide.