I may be beating a dead horse here, but here goes.
For your first smoker, I recommend buying one ready to go, either the WSM or the PBC. My first was a WSM 18.5", and I still have it. It's so easy to produce good food. A buddy of mine chose the PBC, and he also produces really good food. You can set it and forget it, which is important with a family. There are also a lot of things that you don't know about smoking, that you will learn about using a unit that's ready to go. It's an art, and as you cook good food, you will become more interested in it and want to tweak things here and there, modify it, etc. But you're taking a variable out of the equation buying a smoker that's ready to go. If you build one and it wont hold temp well, where will you start with figuring out the problem? Will you ever figure it out? Maybe you'll just get frustrated and decide smoking isn't for you (blasphemer).
I don't think you can go wrong with either the PBC or the WSM, but for my .02, I wish I had ponied up the extra cash for the 22.5" because I hate having to roll my ribs or use rib racks. It's also tough to fit a full packer on the 18.5. However, if money is super tight, and you simply wont be able to afford the 22.5" AND a good, reliable thermo, I don't think you will go wrong with the 18.5. It served me well for two years before I upgraded, and it's still in my arsenal.
Good luck!