Personally I'd get one cat. Because with a pair one will die first leaving the other confused. And if you replace the missing cat, remaining original cat gets cross. In my experience.
Flea treatment: I only do it prior to going into the cattery. Worming: once or twice. Ever.
Holidays: 72hrs or less: automatic cat feeder + one litter tray per 24 hours. Cat locked indoors. Longer: cattery.
Insurance: about £13 pcm. It's with Argos I think.
Vaccines: about £40 a year (actually I do it every 13 months, long story)...
Food: box of 12 pouches of whiskas is £3.50 or 2 for £6 when on offer. Our (tiny) cat has one pouch a day (I think 2-3 is more usual) plus a handful of go cat hard food, which I get when it's on offer. Lots of people think whiskas is junk. But she's a healthy weight, has no health issues and now won't eat other brands. She also eats things the kids drop (crisps, cheese, meat, spaghetti). Water to drink. She loves milk but it's not good for them, I tend to give her the dregs of the baby's bottle if he leaves any. But not direct from the bottle!
I've always trained my cats to come in at night, and then I lock them in. Safer from cars, fights and foxes. Less hunting too. We have a lockable cat flap that only opens for her chip. But she can open it if locked so I block it with the kitchen bin at night.
My DC are 7,4 and 2. We had cats before children, and current cat has been with us since DC1 was 4 months. She's the kind of cat that runs rather than lashes out, and despite a tendency to sleep in cots / pram / bouncer there's not been any cause for concern.