Hey Rosie:
On foam:
I know that DTF works well with 'lectric RC planes -- there's an RC combat plane on Flight Test RC that I think would make a great sport CL model just by leaving off the radio and putting on a bellcrank.
I don't know how it'd stand up to an engine -- I think you'd need a plywood box with the engine mount, then extend the fuselage off of that.
"Real" DTF just falls apart in water -- one of my RC flying buddies has found some foam at JoAnne's Fabrics that looks the same but is plastic coated. He left some under his workbench for a day with water pooled on it, and it showed no sign of damage.
So I think that if it's properly done, water and (if necessary) fuel proofed, it should work well.
On CF and structural strength:
Most of the strength you need in a fuselage is against bending. That comes from having strong stuff on the outside of the fuselage, and the bigger the fuselage the better. It's why so much stuff is built with tubes -- most of the resistance to bending comes from the outside little bit of the tube. As long as your tube isn't collapsing like an empty beer can, the bigger it is and the thinner the walls are, the stronger it is for its weight.
So put your CF veil on the outside of your structure.
It's the same reason that DTF should work if you make a box out of it instead of trying to use a single sheet.