It depends on your skill level. If you're a beginner, I'm not sure you'll find any of Opeth's songs easy to learn. Possibly Benighted, but I'm not sure, what's easy for me could be compoundedly difficult for you. Actually on second thought, if you're a beginner Benighted would confuse the hell out of you.
And To Bid You Farewell, I play that by myself all the time, it definitely works with one guitar. I like to play it on a nylon. I think you're talking about the introduction, the section that uses multiple guitars, there yes, I can understand it sounds like not much is happening with one guitar; but with the right skill level, and enough passion, that one guitar can sound just as elegant as three. After that part, it really hits chords, so one guitar gets the job done nicely.
If you're at say, an intermediate level, I'd say just have a go at To Bid You Farewell and Benighted. It might sound like shit at first, but that's always the case, that's why it's called learning. If, however, your barely learning guitar all together, I think Opeth might not be the place to start at all. I started with Metallica, easy stuff to learn, develops basic rhythm skills and nice metal standard.
You know, about guitar in general, asking for recommendations of what to learn because it's at your level isn't the best way to learn guitar. I remember the first six months of me playing guitar, it was shitty and brutal, I just played stuff I couldn't play for two years. What ends up happening is that by the end of it, your a badass guitar player. Don't be afraid to pick up stuff you know you won't play right. Don't be afraid not to see the results anytime soon, shit takes time. I remember picking up Eddie Van Halen's "Eruption" tab within my first year of guitarplaying; could I play it? HELL NO, but I learned a lot, and now I sound a lot better than I first did.
So as far as learning Opeth, I think to Bid You Farewell is a good place to start, learn all three guitar parts. After you know the whole song, play the guitar part you like the most. The first Opeth song I ever learned was Face of Melinda, and jeez, that opened up my eyes.
and like any good guitar player, you gotta practice, practice, practice...