Men's 'Andoa' Sweater pattern -- tips??

[shemmin]

Hello!
I want to try to knit the 'Andoa' men's sweater (listed in the patters on this website but I think it's actually through Berocca...) but I am kind of having a hard time getting started... I bought some black acrylic yarn, with a weight of '4' (medium?), but the pattern itself calls for some kind of alpaca yarn... Do you think I'll be okay with mine? Does it matter, as long as I get the gauge to match up?
Any advice would be great, especially if you happen to have tried this pattern at all!!
-S