I am not sure what size the tank is, but are you sure you want or need oak structural members? If they are not going to be a visible part of the stand, i would just go with pine. In most cases a very sturdy (easily overbuilt) stand can be built without 2x4 structural members and casework only (with proper joinery). My first "OAK" stand was built with 4x4 legs and 2x4 top and bottom framing, all skinned with 1/4" oak plywood and 3/4" oak trim. It weighed a ton. 2 people can barely lift it.
The green Harbor Freight HVLP gun gets some pretty nice reviews for being a $30 tool. I have an Asturo that I spent maybe $200 on and it does very well with my small compressor.
In any case, Mars Lumber is likely your best bet for the rough 2x4 oak, as it will be dry and straight. The amish and other "mills" in the cherry tree and surrounding area will be hit or miss with regard to moisture levels, etc.