Its been a while since i did any repaints but a decent paintkit is a great start. Honest, its the best way to really get some super detail into your repaints. Its the best way to learn all about layers and how to use them.
I note some PMDG products in your sig. Check there paintkits. They are multi-layerd iirc.
I learned a hell of a lot by painting the CLS 747. The paintkit was truly fantastic and once id cracked how to manage alphas i was churning them out at quite a rate!
Looking at the 'dirt layers' individually its easy to replicate, use the same techniques to 'weather' an aircraft more or even less..i found it the fastest way to learn rather than trying to create things i had no idea about!!
For a freeware paintkit + model may i suggest the tinmouse 737, it really is superb!!
Heres an example of some of the effects attainable with the fabulous paintkit.....
And the finished article....
All my own work on the repaint side. 8-)