Well first off.... GO!
Second off, if you REALLY want to stay on property and make a whole experience of it PLEASE sign up for their e-mail list. I signed up for the Disneyland Paris list and I get e-mails (in English
) on a nearly weekly basis for hotel discounts, package discounts, etc. good deals.
From my research it doesn't seem ABSOLUTELY necessary to stay on property b/c the train service seems pretty fast and convenient but I've never been so who knows if I am just reading a bunch of malarky.
Also, from what I understand 3-4 days would be MORE than enough.
PS: research carefully on restaurant choices if you stay on property. I've heard some not so stellar things about some restaurants there.