I do not use 70% isoproyl alcohol because one never knows what that 30% contains. It's usually tap water, which may contain a host of things that don't play well with acrylic (or other) paints. I only use 90% IPA, and I only use it for Tamiya (or Gunze) paint. I don't use it for Vallejo because Vallejo is not compatible with IPA.

Your cleaning solvent must be compatible with the solvent in your paint.

Cheap airbrushes are just that—cheap, and not worth the money. A good airbrush for scale modeling is going to cost ~$150 -$200 US. Iwata is very pricey, GREX and Thayer & Chandler more reasonable and just as good IMHO.