Hi Dodo,
We've been to Orlando a few times (2AD & 2Ch), and it really is a brilliant holiday, you won't regret the extra cost. We've used American Holidays a couple of times and found them very easy to deal with, no real problems.
We've also done the "DIY" option, which is a little more hassle (not much) but can be a lot cheaper.
As its your 1st time you may feel a bit more at ease getting the travel agent to book everything, but when you think about it, to arrange it all yourself all you need is:
Flights (Skyscanner, Expedia etc)
Accomodation (OwnersDirect etc)
Car (Definitely need this, loads online)
Theme Park Tickets (again, online)
I'd also recommend this forum
www.thedibb.co.uk which is an excellent resource no matter which way you decide to book it.
You'll have a great time in Orlando. There's so much to do and see, I hope you're not expecting to get a rest! Enjoy!!