With regard to how much you'll need...
If you decide to substitute yarns, you'll have to check the yarn called for to see how many yards are on a ball/hank (233 yds is what's listed in the pattern...229 yds is what's given at other sites..., so it's likely a fingering or DK weight yarn...just checked...it's an "Aran" weight). Then add together the number of balls/hanks for your size in the pattern for all the colors and multiply by the number of yards. When you decide which yarn you'll be using, just do it in reverse...i.e. look at how many yards are in your yarn, divide the total yardage you need by that number & you'll have the number of skeins you'll need.