hi mick, I also have a couple of those cheapish (about $30 au) spray guns with the pot on the bottom, they are easy to use and clean, you can use any paint you like through them the trick is in the thinning of the paint, Ive had excellent results with automotive thinners based paints aswell as humbrol and mr hobby, I even use one for spraying release agent when fibreglassing.
The only thing you have to watch is that some have glass pots which will last forever and some have plastic pots which should be thrown away and a glass pot sourced,the good ones down here are revell brand and holding brand, but I think most major manufacturers use the same design and just stick their name on them, I have also noticed that the ones with the plastic pots seem to come from alot of discount auto places.
More expensive airbrushes are great for precison control but I find them more suited to more artistic or graphical work (also great for fine weathering effects)
One other thing is the cheapy ones dont require alot of pressure, I used to use my spare tyre with an adapter.