The game will be (if it ever sees light of day) 2D, top down, tile based: you play as a little guy running around collecting treasure and avoiding monsters. Simple
Plugging IronPython or Luainterface into a .NET project is fairly easy and I can't see parsing XML into a series of singularly liked lists to be enough effort for me to claim the game is more than something simple.
Though I get where you're coming from, for my last toy-project I combined Ogre for rendering and scene management, Mono as a script engine with C++ code and that was not an overly trivial task.
With regards functional programming I feel I've done the majority of risk assessment that I needed to do on it.
For me the biggest risk (and mind bender) was immutability... Before I got into FP the thought of not being able to assign values blew my mind.
But also the biggest pull of FP was immutability, immutability = less bugs, easier parallelism(this project doesn't need that, but its a tool in my coder utility-belt if I can master it). Plus the other things I mentioned like greatly reduced opportunity for null reference, etc.
Another risk, though I suppose its a sub-section of immutability, is the assumed speed hit that constantly creating new instances of objects brings. But this seems to be minimal, I've even seen anecdotal evidence of the Zipper pattern beating out mutable tree structures for speed. Also if I should find my self developing for a platform where the GC is diabolical (compact framework?) F# allows me to fall back on mutable data structures.
Obviously if this where a commercial endeavour risk #1 would be: Where the hell do I hire F# devs from.
On top of all that It's been really fun using FP. I'm less focused on trying to create objects to solve problems and more focused on just solving problems with logical code.
For those reasons I recommend it.
Obviously the flip side of that coin is If your looking to start a new and serious project then you have to consider the time you will need to spend learning FP and the fact you , probably, wont find many people with the skills to be able to help you out, etc, etc.