Hi Murphy,
I would definitely recommend the 12" F4, but there are a few things to consider.
you'll need a coma corrector, precise collimation, the cats eye tools do the job well, a 2.5 to 3 focuser as the light cone is stumpier and you'd have vignetting at the base of the focuser.
it also helps to use a low profile focuser as that reduces the width of the cone at the base of the focuser by moving the focal point inward.
you'll need to leave sufficient room for a filter wheel, OAG as well.
you'll need a larger secondary, and a coma corrector for 2.5 to 3" focusers.
an OAG would be much better than a standard guidescope and guide cam as it eliminates flexure related issues.
for the OTA, it'll need to be very rigid with minimal flexure, and I would recommend either a carbon fibre OTA or a serrurier truss.
i've just completed a 10" F4 serrurier truss and it was inspired by Rolf Olsen's 10" F5 serrurier.
Advantages are very good rigidity, weight savings and ease of fabrication.
my 10"F4 with a guide camera, QHY8 CCD, dovetail, mirrors, etc, weighs 11kg's. I'm replacing the trusses with carbon fibre, so it should end up at 10kg.
Rolf is currently building a 12" F4.7 serrurier as well.
my scope pics are here, pending CF truss.
http://s1289.beta.photobucket.com/us...rurier%20Truss
Rolf's build is here
http://www.pbase.com/rolfolsen/10_in...tube_newtonian
an interesting discussion here
http://www.iceinspace.com.au/forum/s...t=82764&page=4
With your structural Engineering knowledge, it shouldn't be too difficult.
Look forward to your build progress.
James has done a good review of the GSO 12" F4 that comes at a very good price with a 3" focuser.
you could actually buy that, ditch the steel tube and build your own as you'd have most of what you need already.
http://deepspaceplace.com/at12in.php
Cheers
Alistair