I think if he does well, he will want to stay. The question that is going to be come up over and over again is if there is a buyout clause and for how much. If there is one, which I don't see Donovan signing a new contract where he would not have control of his future, then I could see it being for around $10 million, which i think is a little over 6 million pounds. So if Everton can raise the cash, then he will come over permanently.
I also would not worry about another team out bidding you either. LD makes plenty of money, so playing time (for upcoming World Cups) as well as his comfort level regarding his role on the team is what's important.
While I am new to following Everton, I would not think raising the money would be too much of an issue. You get what would be a versatile starting 11 player as well as the face of American soccer. If Landon is a full time member of the squad then Everton's popularity in the states will soar and that is an emerging market that every team wants to be in on. You would think that adding LD would make someone expendable.