difficulty to learn the following Nirvana songs on guitar?

Sounds like you're a beginner, so I'd start with heart shaped box, come as you are, then smells like teen spirit. Those are all super easy songs with easy solos. Milk it and Radio friendly unit shifter are slightly more complex just because of the oddity of the style. If you want to sound anything like the song, you're going to need the correct pedal setup. And they're just weird songs to play because it's basically just controlling screeches and noise.

sappy is just power chords too, so it'll be easy to learn.

Depends on how much time you put into practicing it. Also it depends a lot on your skill on the guitar. I play bass not guitar but I learned both Heart Shaped Box and Come as You Are in a little bit over a day. Smells Like Teen Spirit took me only a couple of hours. Milk It the same. But again its all up to you and how much you put into learning the songs.