have you tried looking into seeing if your area offers low cost vaccination clinics and low cost spay/neuter clinics??? our area has both. and you get a great discount on both things.
Really you need to look into this option.....I took my last 2 kittens to ACT Animal Coalition of Tampa....$65.00 for neutering and shots! They also have vaccine packages that are no where near what the vets charge! Plus they also offer a 3 year rabies shot. I called around when it was time to get my last 2 fixed, the Cat Hospital near me was $198.00 to neuter and then another $112,00 for the shots

There are also other places in the area SPOT is another option that does altering reasonably, of course I know that these are in my area but I am sure if you start researching you will find simular places in your area, but also if you adopting from a shelter most of the time they are already altered and have their first shots....well worth the adoption price.
As for food, I use the scoopable brands and if you shop sales and clip coupons I usually stock up when it is on sale, I find that it isn't near as messy as the clay stuff and alot more convenient.
Most of the time I shop Petsmart for the food, mine get wet in the morning...3 cans split 6 ways, they have access to dry all day long.
People will come here and tell you to buy this brand and buy that brand... but quite honestly I just find a brand that they like that is not full of all fillers...you do not want to buy the cheapest most of time because it isn't really the best, full of ash and fillers. I have always fed either Kit and Kaboodle , Natural or Meow Mix, and their wet is usually Friskies 40 cents a can at Petsmart with the savings card.
I have to second the thought on getting 2 kittens, they have so much fun together.
But also if you are having second thoughts about budgeting for a cat then perhaps it is not time for you to adopt one, since things do come up with vet bills etc. Adopting any animal takes a committment as well as good care and love, animals are not disposable.