The Leper Affinity solo is a fantastic one to attempt. It teaches you a lot about Mike's style, and hopefully that rubs off on your own playing.
I find it's important to focus on learning Mike's solos if you're gonna be learning Opeth, because he has more advanced elements of style and notation. It'll help you be a better guitarist, in terms of your expression, not just how fluently you can play a slur of notes.
Having said that, you should focus on his later stuff. Like MAYH and beyond. He really became a more fluent player at that point and it really shows.