Phil, I'm not a rocket scientist; they are my students.
I started with a Paasche H set brush, three tips....fine #1,medium #2 and large #3. I used this brush for many years before getting a VL internal mix set, the H set is an excellent brush for genral work, it won't give you fine control to paint pencil lines, but it will teach you about airbrushing and let you do almost any type of painting\weathering you want.
I recommend Paasche brushes and the H set is an excellent first brush.
i encourage you to purchase an airbrush as they are very useful in many areas other than modeling. i do recommend that you get a siphon feed rather than a gravity feed as they are easier to use with less problems. external mixing should be the only way to go considering internal mix is complicated and difficult to repair and clean. as i have grown up in my dads body shop with a very good painter, i would recommend an iwata airbrush. for a beginner though, a good airbrush should be the badger line. iwatas can be very pricy, but they are among the highest quality. do not take my words too seriously, though, because i am somewhat new to modeling and am not sure which brand or type is best for trains. and finally, make sure you have a compatible compressor as the price of a good one could discourage your will to take on yet another great hobby.
I have a Pasche internal mix. I have had good results. Sometimes there is an inconsistency in the applications of paint. They like the paint thinned just right to match the needle/tip size and they like to be clean. Too thick of paint and a dried paint buildup will give spitter spatters and/or runs or not enough paint. It is also important to keep the needle tip in good shape. Good DRY air pressure is very mportant too. I attended the Badger seminar in Detroit and sure leanrnt alot about technique and cleaning. Thought I new what I was doing.....ha. I did learn that a gravity flow needs a lower pressure and gives a more finer precise line.
If your seeking just a good overall coverage an no fine detail spraying with ease of use and cleaning I would go with a external mix. You can always mask for detail painting.
There are several experienced users here that will also add some input, perhaps some external users.