Just build it with the lens you've got, and watch out on ebay for something better - it should be easy enought to switch in a better lens latter.
I did pretty much the same, but picked up a few cheap lenses - an ancient ross lens cost $10, and a teletessar about the same. The catch is they don't have shutters. Stopping right down in bright sunlight give an exposure of about 1 second on paper (asa 5-10), so I just put my coat over the lens and used that as a shutter.
Lenses with shutters are way more expensive - they're cheap enough compared to serious equipment, but more than I wanted to spend on a home build project. I intend to build a falling plate shutter into the body of my camera.
for my first go I built without movements so it would be an easy build.