For what its worth, I would not spend your budget on a used machine, especially if it is a transformer based machine. The power requirements alone are significant. Plus, these transformer units are not portable at all. The smaller multi-process boxes are and that is not something to overlook.
You can easily find yourself requested "at a site" to repair something, or someone brings you something that won't fit in the shop. How will you manage to weld it outdoors or on the road with a 150#-300# transformer box?
A good, small multi-process machine would be my first choice. These won't do AC TIG, but, they will support Aluminum MIG with a Spool Gun (which is a very nice alternative, especially on thin aluminum). Until you need it, AC TIG is overkill and a budget issue; hard to get AC TIG and DC MIG boxes in the same footprint for your budget. If you get going, a dedicated AC TIG box is my strongest recommendation. A SquareWave 200 is very decent box, or an Everlast, or Alpha... But having a 2nd machine then will be more cost-effective and supported by your business than having it now and not needing it.
Stick and MIG (and Spool Gun) are probably the most widely used processes for a "Gravel Welder" in a small town. Be proud of it, and good at it!