Anything within a 2-3 hour drive from home is always on the table. Further, and it needs to be real good. I've gone 300, 400, 500 miles from home to fetch stuff, but have begun having most things shipped. I now quote freight for even trips that would take me 2 hours from home...if freight is cheaper than gas, I ship it. Time is tough to carve out for tractor chasing, though I do love the road trips. I also book as many stops as I can, to make the drive worthwhile.
Last March I gambled on the weather, and logged over 800 miles in one day. I left home around 4a, and was in bed by 9p. My stops were quick (one kinda freaky stop in the dark, in a Chicago Suburb, around 5am their time), and I even made a barn find at a house overlooking the Mississippi in Iowa, and litterally 'dug' out this gem. I had no info or pictures on it before going, other than a phone call where the seller told me "it's a big old saw that fit on the back of an Allis Chalmers".