I'm pretty happy with the look and navigation but I do have a few (hopefully useful) comments....
The panel on the left doesn't change my browser's scroll region. ie if I have a small browser window open, the bottom buttons are off the bottom of the screen and there is no scrollbar to get to them.
Maybe some pictures or thumbnails of models (from a random pool/gallery) to liven things up and show exactly what the site's about.
When there is nothing on a page (eg the Parts page) - refresh the page with an 'under construction' notice - at the moment it doesn't do anything and apeears to stay on the current page.
I'm not sure I like the scoll regions - but where the content can vary so much I'm not sure how you'd get around that - maybe it's just a question of making these regions a bit bigger, perhaps by reducing the size of the nav-button region.
Hopefully these will help.
On another note - I hope you don't do too much to the magazine format - I like hardcopy and issue 1 printed beautifully. I don't think I'd like another web-format 'zine.