Silent Storm engine.
http://en.wikipedia.org/wiki/Silent_Storm_engineAdvanced physics model, nearly all structures are destructible, and ragdoll physics is employed for bodies, Three-dimensional mapping allows for obstruction calculations and cover effects from any direction. Bullets ricochet, and their stopping power depends on the strength of the weapon. Modeled are materials' effectiveness at stopping ordinance and visibility based on lighting conditions.
The engine has been used in a variety of games, so it's well documented. Silent Storm, Silent Storm: Sentinels, Night Watch, Hammer & Sickle and Day Watch. And a modified version of this engine was used for Heroes of Might and Magic V. The 3D graphics of which don't look half bad and whose pc requirements are modest 1.5 ghz 512mb ram. And it's old, the publisher who owns the engine went bankrupt in Jan 2011 and was bought out, so it should come dirt cheap. This engine should take care of the nuts and bolts of implementing combat as the silent storm turn based combat system is probably the best system out there, better than JA2 system imo. All that's left is to put in a nice text dialogue system and that's it. Hoping for a Fallout Tactics like combat system though.
Also why limit the game with linux/macosx compatibility?. Linux has 1.11% market share, Macosx 7.31%. Money talks and I'll bet the majority of donators have windows as their primary os. I'd also bet the majority of both Linux and Macosx users have access to windows. Macosx with bootcamp and emulation programs, Linux users as a secondary os.
Also the owners of the silent storm engine, JoWood, bought The Adventure Company in 2006, who have worked with a variety of developers. Recently both have been bought out by Nordic games in 2011, a new company which seems to have few games under it's belt, all basic but moving up as it publishes Alan Wake. So I think the developers of this game will get a very receptive welcome from them.
I will also add that the company who owns the engine has probably seen alot of people leave. That gives the developers the opportunity to hire someone who's already worked with the silent storm engine, and more so on a WWII themed turn based strategy game, not too dissimilar to post apocalyptic as a setting. No better engine support than if it's a guy who built/worked with it for years.