Chris Firstly having re-looked at your pics it is amazing the finish you have achieved using just hand and brush, very well done. I still believe an airbrush is the way to go though, I now hardly hand paint anything on a bike build. It is just so effective for all the metal bits and you end up with such a clean finish as opposed to the inevitable tiny brush strokes when done by hand. I do however understand the budget thing, it's taken me years to get my bits together!
An airbrush needs air to work. When I first started I used to pump up a tyre and with an adaptor on the valve got by, till one day I got tired of going to the garage to fill up with air all the time and seriously over-inflated it. Needless to say the thing exploded in my room, luckily I was not near it at the time! You can use aerosol cans which also work but can get costly. Best is to purchase a compressor, these can range in price from really cheap to outrageous. Best is one with a pressure reservoir which means you don't get pulsing air but rather a smooth flow of air. Can be a bit noisy but they fill up pretty quick and then you work in silence till the pressure drops. As for airbrush model I started with a very basic external mix type and have progressed to a badger single action, don't feel I need to get any fancier than this.
As for paint wastage, yes that does happen but with experience you can judge pretty closely how much you need so I find it is minimal. I only use the tamiya X range of enamels and have never had any problem. I find the magic mix ratio is 1 part paint to 3 parts thinners.
The only down sides for me are the little bit of extra cleaning you have to go through to clean the bottle and airbrush after each spraying. Also the fumes can build up in your model room. I made a spray booth with extractor fan which pipes most of it outside. Here is a pic.
Careful with the Klear, you will see in this build how it can go wrong in some posts by Seabee. test on spare kits or bits of plastic till you have it mastered.