Hey, everyone will have different opinions and there all valid, all I can do is give my 2 cents. Brushes are very important, I personally prefer kolinsky sable brushes. I'm not sure but I think the GW brushes are now kolinsky and are actually quite nice. I have two types of brushes, W&N series 7 and GW and lately I find myself using the GW more then my significantly more expensive W&N. Reason for this is that series 7 brushes are made for watercolor and have quite long bristles. I prefer a little more spring in my brush which I find GW is now better for. I paint almost everything with 3 brushes now. I have a large wash brush from GW which I will base coat with. A standard GW brush which I paint 95% of my stuff with including eyes etc on 28mm and lastly a size 0 W&N series 7. The series 7 are deffinetly better quality with finer sable but I find for small miniatures the bristle lengths on the GW are superior. I have also heard alot of good stuff about rosemary and co. but never used them myself. Where I am series 7 start at $16 canadian for a triple 0, are about $40 for something slightly larger then a wash brush, and about $200 for something slightly larger then an old GW tank brush. I'll also add brush care is incredibly important. If you want your brushes to last never compress the bristles, never leave your brush bristle down in water etc, and I would also suggest getting some brush soap. I am not the most careful person with my brushes but I do my best and I havent bought a new brush in about 3 years. My W&N brushes still look like new but my GW brushes are still more then serviceable though do show some slight wear.
NMM - I personally would not mix NMM and TMM. This is just my opinion. You can get alot of control out of metallics depending on your style but no matter what they will always pick up more light then flat paint, or I think they will. NMM done right offers complete control over your light source, that being said even if done seemlesly I think TMM looks better on the table top then NMM. NMM best comes out in display pieces. As for an easy recipe there is'nt really one. If your light control is done right you can use any color and have it look metalic, this is the hard part. Seemless blending doesnt meant your piece is going to have good looking NMM, perfectly blending grey is easy to do, NMM silver is far harder. You have to think long and hard about where your light is coming from, how its going to reflect, where your hotspots are going to be etc, to get realistic NMM. If done wrong it just looks like something highlighted grey or brown or blue, whatever colors you went with. Light doesn't react the same on reflective surfaces as on matte surfaces so you can't just highlight something normally and have perfect blends and expect it to look metallic. Thats a bit of a ramble and just my 2 cents which may or may not be helpful. If you do decide to try NMM i suggest you take a few minutes to look at your model, decide where your light source is, and think about how you think the light will react on your chosen surface. If your looking for inspiration I would check out the following painters gallerys, to name but a few. Razza, Brokenblade, Yellow one, elly3438, and maybe check out automaton aswell to see what you can achieve with TMM using NMM concepts.
Wet Palette - Probably the easiest way and I think it's already been mentioned is to take a sponge, common kitchen one works fine, place it in a shallow tupper wear container, soak it, and cover it with bakers paper. I suggest a tupper wear spo you can put a lid on it and chuck it in the fridge. I've kept mixes going for as long as a week this way with acrylics.
Don't know if any of that will help but hopefully there was something in that ramble, best of luck, happy painting.
Shakes.
