Alpaca yarn can feel glorious. It often comes as 2,3 or 4 play and 3 ply is generally 'sportsweight'. Worsted will be a 4 ply and knitted in say 8/7/6 US needles. I'd do a small swatch and see what gauge you get as you consider what pattern. If you do a google search you will locate patterns that give you gauge sizings and you will probably determine more exactly what you have that way. What is so lovely about Alpaca is that quite often the colour you see is entirely natural.
The best thing is to determine something called Wraps per Inch of the yarn. At the very bottom of the page I have cited below it explains how to determine the number of Wraps per Inch and what yarn category the numbers mean: