The weathering is entirely up to you Gav, at least as far as how much you want to do. The vehicles that landed on D-day were newish, well kept, and recently maintained vehicles. So they'd be in pretty good shape paint wise. Not many chips or damage. 
An ambulance would have arrived a few days after the beaches were secured and would be fairly well used almost immediately. So mud, dirt, dust, etc would be the order of business that I would choose.
As for paint colour, for most US vehicles I've been using Model Masters ANA613 for the Olive Drab colour and filtering it until it gets to the shade that I want. I really like the look of the colour.
It's and old question though... how olive is your drab? haha. Modellers debate about it all the time.
There's really no right answer though. The paint wasn't all that tough and it was really matte so it faded quickly and changed colour when it got wet or damp. When you add dust, mud, and start the chipping it gets even harder to tell.
So I would pick a colour in the paint line that you prefer that matches your references as close as you thing appropriate and go with that.
 
