Well, i can't help here, it is your decision at the end, all i know is that the more the number increasing in Canon bodies models, the less advanced it becomes, 400D could be an older camera, but it is slightly more advanced than 1100D camera, but why not pass those and go with 450D or 500D, i think 450D is very close in price to 400D if not so much.
I really didn't understand your point about if you will use DSLR a lot, why can't you borrow one from your friends or rent it and give it a use for 1 week, instead go with any advanced P&S new or previous generation one, if that is also not sure to get you shoot a lot then simply don't do photography, your mobile will be enough.