Although you can find the materials cheaper than Dave Roots, The kit Dave provides is complete with tools and materials including detailed instructions and for me that was worth it. Also note there are 2 thicknesses for the spacer/seal. Call Dave Roots and he will ensure that you get the correct one.
I did several Se-Gi windows. The hardest part was separating the panes of glass. The windows I did had a metal spacer in the seal. Cutting the spacer/seal with the metal was time consuming, involving more than a few Band-Aids.
They also recommend a power caulking gun. I would follow that recommendation, appling the sealer with a manual caulking gun would be challenging.