You'll definitely need a car to visit the space centre! most villa communities are about 45 minutes drive from the airport, and as your DH has never driven over there before, check out
http://www.kissimmee-us192.com/british/driving.htm for a few helpful pointers!
You've had the best answers above, but just to elaborate slightly on a couple:
ESTAS - you have to register your details with the US government via
https://esta.cbp.dhs.gov/esta/ a while before you travel. This costs $14 per person, and lasts for two years. This gives you permission to land at a US point of entry, but as with all customs & immigration in any country, it's down to the officer who stamps your passport whether to actually let you in or not. There is a whole thread on this forum for ESTAs, and a sticky at the top. Read the questions carefully on the ESTA site, and if you feel you need to answer YES to any of the questions, come chat to us on here before you do, as answering yes is an immediate need to apply for a full on tourist visa....
Looking at a villa, you'd want to consider pool heating, this is always extra, and averages $100 a week.
You will likely find your best deals by using a private owner for the villa and booking through someone like uscarhire.com for the car, then all you need to worry about is the tickets and flights. Tickets are cheapest bought over here, if you want to hit all the parks then you'll be needed a 14-day Ultimate ticket for Disney, and a 14-day FlexTicket Plus to cover you for Universal's parks, Seaworld, Busch, and Aquatica.
If you want to do Discovery Cove this is totally separate, and in terms of cost if you're doing it it works out best to take their plus package with 14 days access to seaworld and busch and then buy 10 day universal hopper passes instead of the flexticket.... but you wouldn't get aquatica access in that.
Parking is always charged at all of the parks, with Disney if you visit more than one park in a single day you just show your parking ticket from the first park at the booth and they wave you through. Parking is free at Downtown Disney, and you get 3 hours free parking at the Disney hotels if you are visiting them for a nosey or for a meal reservation.
Hope that helps, and keep asking questions!! It doesn't matter if it's been asked before, we don't care and you'll always get a host of answers. Any excuse to talk Disney!
