I am not really worried about parts accessibility (as far as speed) - not up front, anyway. I would first want to know if it is a brush that I like, and would use.
I have plenty of others that I can use day in and day out - So, if a new brush is one that I find I would like to add to my arsenal, it will fit into one of a few categories. The first would be a specialty brush - A brush that I find works well for certain things, but not something I would use for every illustration - these I tend not to worry about keeping parts for. Next would be a brush that I like a lot, but might not fit into the category of a daily user, so I would go ahead and order at least a few needles for it. Then there are those that I just love using. On any given project I have several brushes that I would grab and enjoy the heck out of using - these ones I tend to keep multiple spares on hand. Needles, nozzles, seals, etc.
I am also in the unique situation that time is not money - I am basically retired, being a full time caregiver for my disabled wife. The airbrushes, and painting is just something I do to pass time, and free my mind. I am simply working on accumulating an inventory of pieces that we will probably go ahead and use to start selling prints, and perhaps do an occasional show or two. Being in this situation allows me to try things that many others may not have the time to... It also allows me to share what I have experienced with others that may have similar interests or curiosities.