I don't think its particularly unfair in your circumstances to have cats. Lots of people leave their cats overnight without any problems, particularly if they are very independent ones. One of mine would tear the house apart and the other would cry all night if they were left so they never are (I would be too stressed out too, I hate them being on their own), but then I have people who can just come and stay at the house for a night if needed. They are on their own while we are at work (8-7) and that to me is more than enough (again my opinion, it does entirely depend on the cat) as they need lots of interaction (even the older one). If they are left alone make sure they have plenty to do e.g. toys, things to climb on etc. one of ours just sleeps but the other one can be a nightmare when she's bored!
Be careful with insurance and do lots of research, our younger one who is almost 5 is only about £7 a month with a low excess. However the older one (just turned 12) is £25 a month with an excess of £125 plus 20% per condition/claim, which will only get higher as he ages, but this is for a lifetime policy.
We spend about £20 a week on wet food (they also have raw meat from when we're doing our own meals) and their dry food is usually every few months as they take ages to get through it. Bear in mind that any health problems could mean them being put on special diets which can be costly, but bulk buying can sometimes make this easier. Litter too can be bought in bulk to make it more cost-effective (only one of ours generally uses a tray anyway, the other one prefers the garden lol).
Generally the costs of keeping a cat aren't too bad, the worst bit is the unexpected vet bills, which if they are wanderers will be more likely, my older boy, Timmy, lost a leg by getting into another garden and getting attacked by a dog, thankfully this was before I adopted him, I can only imagine the vet bills on that one! My old cat managed to get his tail run over twice (two separate occasions - who says cats are intelligent!!), this was then removed and my parents didn't have insurance for him so they had to pay the lot.
I would second what has been said above about adopting rescues, both of ours are and rescue centres would always be my first port of call when considering a new addition.