Dave,

I've worked on HL/QuakeIII mod projects before (3D Modelling) - although I wouldn't know the first thing about the HL2 engine.... :|

So..... a quick message to all the haters out there....

HL and HL2 have one MAJOR thing going for them.... Hundreds of thousands of users that love the game and have developed 'something' for it using the in-built engine.

That gives HL2 an overwhelming developer-base along the lines of Linux and other open-source projects, and yes, even MORE developers than Microsoft.

I say KEEP GOING WITH IT!!!

There are thousands of physics freaks out there who would love to help you incorporate real-world physics with what the HL2 engine will give you to work with.

I'd say a great place to continue research for your project is to cross-post development related issues in specific HL2 developer forums, they ARE out there, full of lunatics just waiting to sink their teeth into a new project.

Even if you can't get the physics perfect, you'll end up with a great arcade game - and maybe a contract with Vivendi Universal

Nathan.