Hi Shahgazer,
Sounds like a great project and you are looking up some good references regarding the design. I really like the minimalist designs too. I built a 12 inch flex rocker design after finding the conventional truss style too hard on my back.
I think a predominantly aluminium design would be quite feasible. Then use marine ply for structures like the ground ring (if you go for for the flex rocker).
The thing I learnt from mine is that getting good motion is a little trickier with a flex rocker. The traditional ebony star and teflon did not work well - it became very stiff. I think this is due to the very large size of the bearings. I replaced three of the teflon pads on each axis with roller bearings (and left one teflon pad on each) and replaced the ebony star with aluminium on on the alt axis and smooth formica on the azimuth. Now the motion is very smooth, except in dusty environments when dust gets into the teflon block on the azimuth axis.
I found it easier to use a central pivot for the flex rocker than use a hollow ring, like with the tri-dob design. It also makes it much simpler to mount encoders later. Dale Eason's design shows what I mean:
http://home.comcast.net/~doeason/16inchscope.html
I used 6 truss tubes on mine and it works well but they are much less forgiving of errors (than the usual 8). You need to close the truss triangles at the mirror box and the upper ring carefully or the truss arrangement flexes as it moves.
If I were you I would go for the 16 inch mirror and I would prefer an F5 to an F4 (better images and better chance of good optics). With a minimalist design you can keep the eyepiece height relatively low and should be able to avoid too many steps.
I have attached some photos that I hope will give a few ideas.
Hope that helps and good Luck!
Rod.