Not sure where/why people are confused about a DCS P-51D "fitting". Just like the DCS games before it AI opponents at first with co-op multiplayer, or combat with other human Mustang pilots. Subtract SAMs and and maybe BMPs you'll have plenty of viable ground targets. Even them building WWII era ground units probably wouldn't be too tasking. P-51Ds engaging CAS level jets isn't even too far fetched, given P-51Cs successfully engaged and destroyed ME-262s. Which on paper (and accuracy of wikipedia) was quite comparable performance-wise to A10s and SU25s.