Apologies to all for resurrecting this thread...or indeed adding to it...but...
My only advice would be this...
If you have a good spot where you can shoot aircraft on the ground then start with a cheap (but quality - there are lots around from Canon / Pentax / Nikon) used SLR with just manual focus - this will also allow you to get a modest selection of lenses for not big money. You will get shots good enough with this setup.
If you are limited to shooting moving aircraft (approach/takeoff shots) then consider autofocus, but still look for a nice used body and maybe get a good 70-210 zoom as this will tend to be smaller, lighter, and faster (larger maximum aperture) than a 300mm zoom.
After that you need really to learn and understand the scan techniques for your chosen film (print/slide) as this will be where you might find it gets difficult.
But remember, you do not need to spend lots of money on camera gear - there are thousands of perfectly good used bodies and lenses out there.