I use the 370..... it's simple, double action, internal mix sold at Walmart (and others) in a set for less than $40. It's compatible with all the accessories for the other Aztek brushes.
It's been a very good airbrush for me. I use it and a Paasche VL equally.
As far as thinning- each paint will be a little different. I've read it should be about the consistency of milk. Sounds silly, but I actually put some milk in a paint cup and observed it. A simple test is if you dip a stir stick in it, and put a drop on the sidewall of the container, it should flwo down fairly freely.
If you're using it with canned air- I had the same problem with the paint running. Turned out the can just blows waaaay to much air through. I almost gave up on it. A compressor with regulator will really help. At 15 psi, it works great for me- acrylics and enamels.
As far as having the handle pulled down and all the way back, if you're using the nozzle that comes with it, it should be spraying like crazy. It may have been the acrylics dried up in the tip. Try this- soak the nozzle in Windex for half an hour or so. While thats going on, gently clean the interior of the brush by spraying some Windex in it, and run some water through it. (Not airbrush it- just run some warm water through it.) Fill the siphon jar about halfway with Windex, and then put the tip on. Spray the windex through it, and that should clean the tip out.
What I did with my Aztek was to
buy several of the side feed gravity fed cups. When I shoot paint, I use the side feed cup. If I'm switching colors, or not going to be brushing for a few minutes, I attach the siphon feed jar (already filled with 50/50 Windex and distilled water) and blow some through just to clean off any paint. Be sure when you resume painting to blow a little color through to clear the water/windex.
When I'm done, I blow Windex through the brush, then wash the tip, cups and body of the brush in Windex, then finish with warm water and some mild soap.
NOTE: The Windex and water are only for acrylics. Switch to airbrush thinner for enamels, although you can do a good final cleaning with Windex even with enamels.The key with an Aztek is to clean it thoroughly each time you use it. I find it takes me no longer to do it than my Paasche.... in fact it often takes me a bit less.... not much, though.
I try to stay out of the whole argument over Aztek vs. other airbrushes. Like I said- I use both and I've had good experiences with both. Now- I've only been using them a few months, but so far the only problem I've had has actually been with the Paasche. And it was my fault- poor cleaning.
Practice is what will really help. Practice on a gallon milk jug, or even slap together a small kit just for practice.
If it will help, PM me and I'll be glad to help you. I may not be able to answer advanced questions- there are folks on here who have years of great experience, but I'm fairly new to this, so maybe I can answer from that standpoint- share the "pain", though it's not really a pain. It's actually quite fun.