The rep restriction on this is obviously pretty sensible, but the other two aren't really imo.
Users are used to be seeing the accepted answer at the top of the answers. Regardless of who the author is the right answers should be presented first.
The time restriction doesn't make much sense either, there's nothing to gain from posting and accepting your own answer straight away, in fact I'd probably down vote a question if I thought this was being done.
In cases like thisthis where I've put up a solution to the original problem as the answer a few hours later I don't see why it can't be immediatly marked as accepted, and placed at the top of the answers list for anyone else who reads it.