I finished it!
I kind of rushed it because next Saturday there is a local contest and I want to take it there.
Every time I sat down to work on it I kept thinking "this is a project that can easily get out of hand don't let it" Because there is no limit to the level of detail, modifications and accessories I kept it simple and didn't worry too much.
I used the raw umber panpastel and it worked like a charm. All the exterior and wheels got a dirty look in 30 or so minutes.
The "turret" is made of warhammer 40k parts (the ring), a couple of .50 cal from an Academy MG kit (I figured they might look like .30 cal in 1/24 scale) and sheet styrene.
The chasis-exterior fit was horrible and I realized I'm not a good car builder after gluing those and seeing how tilted the model is. I guess I can say the cannon is way to heavy and affected the suspension ;D
I used AK's winter streaking grime for the underside and different rust products (the one that I like more was Modelmaster's rust color).
All in all I'm happy. I think it looks mean and taking into consideration it's my first car model and the first time I scratch-build complete parts I feel satisfied. Still I feel I need to be more careful and don't expect any awards on Saturday. I'm already thinking about building another PA vehicle, maybe next year, but this time more ambitious and fewer errors.
Later I will add a post with some word on what worked and what didn't.