Actually, P90 first seems like a good idea. But if you actually want to money to be able to grind up to P90 faster, then go for the Streyr M. Since well, it automatically unlock it in your first play.
Use the golden skin on your stery m, cause that's like the first weapon you will always have as a beginner. Later, use the golden skin on the winchester as you can literally oneshot any zombies and recieve satisfaction (ofc not a ticker to the body but you know what I mean :] ).
Seriously, the golden kill effect gets in the way too much once you use it with P90 because its the type of weapon that kills multiple zombies fast, I suggest the Rambo knife because you wouldn't get much of kills with it, it mainly used for tripping, then use it on P90 when you feel comfortable.