Which one should I start off with??

[bladerunner900]

One last question...

if I get both P-51 programs, MIL and CIV, how do I load Accusim? Is there a order in which they should be loaded onto my computer? Thanks for all the help!

There is an order. Install the main aircraft package first. This can be either the civilian or the military version, or both. Then install the base Accu-sim pack. This will apply Accu-sim to either product or both products at the same time. After that, follow the link Lewis posted to update your P51 Accu-sim package to the latest spec.

If you get the civilian now and the military version later, you will have to run the base Accu-sim package again and then run the updater again.