I think upgrading from an Aztek is always a good idea. Although I'm not personally familiar with the Iwata Revolution, given that the venerable Paasche H single action is more than adequate for 1/35 and 1/48 armor, I should think the Iwata would more than meet your needs. If price is a consideration, you might want to check out Badger 100, 150, and 155 Anthem series.
About the air cans, you will really thank yourself later if you lose them in favor of a compressor. One of the tricks to painting well is having your air at a proper pressure. Without a gauge, you're really guessing at your air pressure and as you use a can, it cools down which causes the pressure to drop as you're using it.
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Andy