It can be as complicated or straight forward as you want it to be. There is a lot of information online and it can be a bit overwhelming if you spend too much time reading around it. .
If you stay onsite you have access to an extra hour in the parks each day and the convenience of being close to the parks. There is transport that will take you door to door, although you can walk to all onsite hotels easily enough. Onsite hotels also have cute Disney stores and will also have characters dotted around the grounds or in a particular place each day - which your DD might like.
Onsite hotels also come with tickets to the parks for each day you stay. EG, if you stay Friday and Saturday, you get tickets for Friday-Sunday.
I'd recommend not visiting during school holidays or weekends if you can help it, as the parks get so busy, but you may be restricted with school. Try and avoid French holidays if you can't avoid UK ones.
Partner hotels are located fairly close to Disney and also have shuttle busses to the parks. They don't have the same Disney extras and although they're fine, they're not quite as luxurious as a Disney hotel - however, they can be good value.
Saying that, as I mentioned above, when you add on the ticket prices, it often isn't that much more to stay onsite and have the onsite perks.
If you book direct, you can buy flights, hotel and tickets all in one go. The meal plans are a good shout as the breakfasts are fantastic (if you like croissants and nutella!).
Sometimes it's cheaper to book flights and accommodation separately. So that means booking hotel and tickets online direct, then just add your flights/whatever transport you opt for independently.
Everything else doesn't really matter. The Dibb forum is great ( as mentioned in a PP). However, there is so much info to digest online, which you don't really need. Just turn up and have a great time and you'll figure it all out. Next time you go, you'll be able to dig a little deeper. Because you will go back!