How hard is it to replace the engine mount and transmission mount on a 2000 mitsubishi eclipse ?

I don't think it's gonna be that hard really. Even if you're going to replace all mounts. You just have to replace them one by one using a jack lift. Actually, I just had mine replaced. What I did is jacked up the engine at the suggested mounting points in a way that the first mount I'm getting out lifts up. They're actually just bolt on so it's easy to remove/install new mounts. Don't remove all at once as you might need to lift up the entire engine/tranny assembly.

yes its hard yes replace all four if your going to do it