For your final build you will want to go with the maximum number of drives you can afford because you can't add drives too increase your storage size, you can only replace all disks of a greater size to increase storage. Raidz2 supports 4,6,8,10 drives with reports of 6&10 being optimal and can survive the loss of 2 drives without data loss. Raidz3 gives even more disk failure survival at the expense of capacity.
I opted to go with Raidz2 containing 4 disks. I originally built a non-optimal pool with 1,1,1,2TB drives. (My pool shows as though I have four 1TB drives since it bases your storage on the smallest drive in the pool.) I have slowly replaced the 1TB drives with 2TB as I get the extra cash. I have one more drive to replace before my pool will be a full-sized 2,2,2,2TB pool. This is my situation, this is all I can afford but I'm not sacrificing my data by being thrifty.
Another thought, you will want to buy an additional drive of the same size to keep it on your bookshelf when a drive dies so you can immediately replace it without having to wait for another to be shipped.
There was something else to consider but I lost what I was going to say.