I'm no expert, but I do have two cats who I've had since kittenhood. My thoughts ...
Will they trash the joint? Not necessarily. Ours have scratched both sofas fairly badly, but if I was more inclined I would have put more effort into stopping that. I thought they hadn't done the classic kitten thing of climbing the curtains, until one day I realised that tiny pinpricks of light were shining through the blackout linings of our bedroom ones! You'd never know though, apart from when it's sunny outside.
Litter tray - we had to have ours for six months, from getting them to letting them out. We still have it, as it's a covered one and sits in an otherwise unused spot, and it's handy if we have to keep them in for some reason. But then, ours never looked back once let out, whereas some cats need encouragement to do their business outside.
Male cats are supposed to be more affectionate, but there are no guarantees, and always exceptions - ours are boys and not terribly affectionate (although one will occasionally decide he loves me very much indeed and would like to sleep 2cm away from my nose).
Ours came from a friend of a friend - I put out a few feelers and had a few possible leads from people who knew someone with a cat with a litter on the way. I'm not sure I'd go down that route again - the woman I got them from misguidedly thought a female cat should have a litter before being spayed, which is wrong. Fortunately when I got there the mother clearly HAD been spayed, but it would have been horribly to have turned up and found an owner who couldn't be bothered to spay her cat, and realise I was validating someone's stupidity. I would look at a shelter in future - whether that route would work for you might depend on DS's age. I looked at one shelter which would not place cats in homes with 'small children' - turned out their defintion of a 'small child' was 'under 10'!
Kittens ideally shouldn't leave their mothers until 12 weeks - mine were 8 weeks though, and have no problems I would attribute to being taken from their mother too early. They settled with us very quickly - I think having each other helped.
You don't need milk - a kitten that's old enough to be away from its mother has no need of milk. I would get one bed for two kittens - cats notoriously refuse to sleep in anything you've provided specifically for that purpose, but mine did curl up in their bed together while they were still small enough to do so. Scratch posts - yes, but with caveats! We got a little one which they liked when small, and then never used (too small). Then we got a big cat tree, which they climbed a bit, then ignored - NEVER scratched it. The big success was a scratch mat, designed to be used horizontally, but my cats are not horizontal scratchers, so it's tied round the bottom newel post on the stairs, which is one of their preferred scratching locations. You also need a cat carrier for bringing them home with you, and then for taking them to the vet. We have two, as they do sometimes go to the vet together, but I know people with two cats who manage with one carrier - you'd only need one at first, while they're small.