FWIW I paid $700 for my '75 816 and 50" deck when I bought it a number of years ago. While it ran well, and everything worked, it didn't look as nice as the one you're looking at (which is why I painted it). Reading your post, it sounds like it is a nice tractor, and has been well cared for, so $700 doesn't sound like an out of the ball park price to pay to me. To me, the year they were built doesn't matter as much as the condition they are in. If it's in good condition like it sounds it is, it is a tractor that, taken care of, will give you many years of service.
There are many attachments for these tractors from various mower decks to plows, snow blowers, and even trenchers. You can also use attachments originally designed for 2 wheeler tractors (walk behinds) including brush hog mowers, sickle bars, rotary plows, etc.) using a front adaptor kit. Since you mentioned brush, there are even McKissick shredder/chippers that were built that attach(ed) to Gravelys. They were designed for 2 wheelers, but again, they can be used with riders using a front adaptor kit.
OT, my personal favorite attachment is the Gravely ice cream maker.
The PTO (shifter) is under substantial spring tension when the PTO is disengaged, so when you test the PTO (engage the mower deck), move the PTO shifter slowly (you will have to hold it back as you allow it to travel) until the mower begins spinning up. Then you can drop the shifter the rest of the way. The mower deck should begin turning (pto engaging) around, or slightly past the half way point of the pto shifter's travel.
And these are the beginning, and ending serial numbers for 1972 tractors (limited slip diffs.): 10878A-15044A. Rumor has it that a few early 1973 models may have also left the factory with LSD. Not that it's the end of the world by any means if it isn't. My non-LSD tractors do very well under all circumstances. Good luck this weekend!