Hey Edog,
Couple of quick answers to your questions.
1.) References, references, references. Look anywhere and everywhere for pictures of the tanks you are going to build. Seeing as you are on the internet, there is a wealth of info out there....digging it up can be fun though. Here's a good one to start with AFV News www.activevr.com/afv/ This site has lots of references to view and tons of links. I also highly recommend the purchase of any of George's scale drawings....good quality, low price, and some obscure choices to boot.
2.) Want to make mud.....check out Ricky Chin's mud making secrets in the photo gallery of the Rocky Mountain Model Club. www.rockymountainmodelclub.com/main.htm
Otherwise experiment with color, white glue, putty, baking soda, organic materials...just about anything. Try it on an old kit or part that you have lying around. Don't forget to try pastels (chalk) to give it that dusty look.
H.M. & G.B.
shermanfreak
Happy Modelling and God Bless
Robert