It looks very good so far.

Perhaps Mig pigment, pastel or colour to create mud effect on figures

Yes, I thought about mud spashes on the figs, but I´m still not sure about the best way to do it. I really don´t want to ruin the paintjob with just brown colour as mud. I´ll try some techniques and decide which one I´ll use...

A good method I have found to create a muddy effect is to use very thin middle brown paint...dab it on here and there and then before it dries wash it wth just water...this removes a lot but "smears" it about. Then do the same with a lighter brown.

Again a graet work , i use Revell dark Earth for dirt Effects , and i would prefer a carefully Drybrush than using much water to avoid getting color on areas it wasn´t planed . But that´s just my Opinion , Pauls idea is good too , but needs more skills .