The only "just for sim" paint/texturing program is Visual Aircraft Studio - BUT - that only works on the FSDS source file ...  you'd still have to produce the airplane.

MS Paint is "fine' for touch ups, but as you've found out, other paint programs are more versatile.

Any good paint program - I favor Paint Shop Pro - is perfect.  The key to repainting is understanding the different formats in use. and using the right converter.

MArtin Wright's DXTBmp is the flightsimmers "standard".  MS' own "Imagetool" can also be used to do the conversions.

Key to any texturing, I think, is a paint program that allows you to use and manipulate layers, and offers you a smorgasboard of tools and effects.

Among programs that I've seen used by flightsimmers are:

Paint Shop Pro; Adobe Photoshop; Corel Draw; Corel Paint; Gimp!, pixia ....