Well, if you going for a new FS game, you might as well aim high and miss low, than aim low and miss alltogether.
We're using the CS engine. It needs some work to make it how we need it, but shouldn't take too much effort.
I took a look at Torque. 1 big problem with it. Any game you make, HAS to be published by them...read the fine print in the EULA.
D4 is shipping with a complete modding tool we call D4Edit or DIVE. If you tack on FS3 to D4, making mods would be incredibly easy.
As for OS support...hehehehe.
CS supports:
Win32
Linux
BeOS
MacOSX
PS2
DOS
OS/2 Warp
UNIX
Other *nix flavors
(Did I miss any?

)
That's actually part of our current problem. To MUCH support for too many things. CS has over 30 dlls...that will be fixed for each platform release.
I'd love to help you make the real FS3, and I do think it's possible, but I won't say it'll be easy. It'll be part of a retail package, and as such, you'll have to learn project management from the level of a Game Designer. You'll be expected to write documents, set goals and timeframes.
Game Design isn't all roses and butterflies, it's hard work. If you're not ready to take it on and stick with it even when it gets stressful, don't even bother to think about trying.
BTW, thanks for the big welcome all! It's nice and refreshing from spending time on the DBB where I post any news of D4 and a flamewar breaks out.