The Monogram Spitfire Mk.II is a good choice for your first Spitfire build. Maybe not the most detailed Spitfire kit out there, but it builds up nicely and, as you already mentioned, is pretty cheap in price. As far as pre-shading is concerned, try not to use black if possible, instead, try to use a dark grey such as RLM 66 or some of the Floquil blacks such as grimy black or oily black, which are actually dark greys. Dark greys leave a more subtle and realistic shadow. As far as thinning, 75% paint to 25% thinner always works fine for me. I use enamel paints though, I have no idea if this thinning ratio would work with acrylics. Anyway, despite whatever glitches you have experienced so far, your Spitfire is turning out nicely.

Good. Seeing that you use enamels, the 75/25 ration will work well. That ratio works even better with Floquil enamels as they are somewhat pre-thinned to begin with. About the only acrylics I ever had any success with are Tamiya's. The only problem with Tamiya acrylics is you have to mix them most of the time to get a correct shade of something. That can get costly. Anyway, your Spitfire looks good so far, please post in "Headlines" when you get her finished.