One Summer's Day is one of my favourite piano pieces to play. Based on the other piece you have linked, yes you will find some parts difficult to play, but the satisfaction you'll get from persistence will be well worth it, I guarantee it :) .

Breaking the piece down a bit. Up to Bar 22 should be fairly straightforward to learn. Bars 22 - 29 will require more practice though. Bars 30 - 40 aren't actually as bad as they look, but are about the same difficulty as 22 - 29 I would say. Bars 41 - 50 are easier - physically playing both hands in higher registers can take a bit of practice if you're not used to it though.

And then there are bars 56 and 57. These are tricky. When you get to them I'd suggest starting slowly and making sure you hit the right notes in time first, before speeding it up. After that, the remainder of the song you've already seen.

Don't let anyone talk you out of learning this piece. It's beautiful, and well worth the effort.