Coaster brakes work well, but it's all in what a person would rather have. I personally have only built two bikes so far and both are coaster brakes. The advantages are only in the simplicity of it, but I do know that if a guy wants to put one of Sick Bike's shifter kits on it, it's better to have a multi-speed for more gears.
The consensus for frames is NOT to go with aluminum, however some members do have them. A number of members have the Schwinn bikes, and if I didn't get my Greenline, that was my next choice.
The nice thing about these kits is that you can build any kind of bike you want. Some guys go all out and build a pricey bike for their first build, but you really don't need to and can build it up as you go.
I, personally, would get the Schwinn, build it and work out the bugs in it and then keep an eye out for a nicer set of beefy wheels and just go from there.