Really..you just pasted the same question ...
"I want to be able to master both portrait work for airbrush and also create fantasy art work like magic the gathering with the airbrush."
1) Are professionally trained in illustration work or fine art?
*** You need to realize there is no different in the techniques used for fantasy or portrait work, when it comes to airbrush skill levels.
"use the airbrush fro background work - however I would like to eventually only need to use the air brush to paint completed works."
1) For background work a simple Paasche Mdl H would work fine. I set up Disney Background Artist some 20 odd years ago with the "H" and a Sparmax 250 Twin Cylinder compressor. A couple got VL's with #1 tip/needle sets. Personally I have 12 airbrushes at my work station, but for the most part I used 7 different mdls, 4 different brands; EFBE, Paashe, Sparmax and Iwata.
"the HP-C plus is a workhorse standard "
1) I was the iwata service agent years ago and that statement is news to me. I think it has more to do with your location on the planet when it comes to brands, as several companies make great brushes. The choice has more to do with availability of spare parts, price, action type (single, dual, progressive) internal and external mix and most important; the feel of the brush in your hand... keep in mind you will be holding it many hours a day.
"11 by 14 for portrait and 18 by 24 for fantasy style art work "
1) The final print size usually determine the artwork size. Such as I used A-5 board for book markers as a sample. Magazine images and center folds I usually used A3 or A4 CS10 (no longer for sale...I have got a few boxes for my own stock).
As a final note I have ran workshops, classes and seminars for professional artists and illustrators till about 13 years ago, (wounds from Vietnam caught up with me) so I retired from full time work and now work from my studio.
To reach the level of skill to produce quality professional level work takes years of pratice, but the finish results can be great.
Here is a sample of one of my students from the late 90's, this was an Illustration for a "Computer Gaming" magazine and done on A3. At that point in time, the computer work wasn't the quality (yet) of a good illustrator. His name was Jamie Tuffrey.