Do these have to be reprogramed if you buy a used one?

 

Depends on the fuel injector rating. if your car uses 19lb injectors, then you can use a mass air meter calibrated for 19, 24 for a 24, and so on. If not, then your car will run alittle crappy.
If you bought a used 24lb mass air meter and your's is 19lb, you can change out the electric connectors on top of the mass air meter, just swap them out . But your's is a 80mm mass air meter and i believe the 2000's still had 19's and not 21's yet. Are you getting a bigger mass air meter? cause 80mm flows plenty enough already.

other than that, you will have to get it calibrated.

 

I would be suspicious about anthing this sensitive being sold used.If it's good it's good,but if it's not so good,,,well,,,most people won't accept a return on anything electronic.Caveat Emptor!!!

 

99-04 stock mass air meters are good enough for a bolt on mustang right? Say cold air intake, larger throttle body, and plenum....the stock one should work fine?

 

they are fine up with every bolt on with cams and all. Just once you push boost threw them you might wanna upgrade, depending on the situation that is.