Mark,
That Ford 1700 was Ford's replacement for the old 9N and 8N that farmers used for years and years! It is a good tractor, and is capable of a lot of work with the two cylinder diesel engine, with a few rev's on it! You will run out of traction before HP! You only stated working 5+ acres; unless I am missing something?
Don't base the condition of the tractor, or the engine on the black oil; that two cylinder diesel engine is an indirect injection,...the oil will turn black shortly after any oil change; even when new! Just the nature of the beast! We have two of them, my son has a 1700, and I have the smaller 1500, that I have owned since new 26 years ago. The only problem I have had, was replacing a water pump, after thousands of hard hours.
IF the sheet metal, clutch, loader, and three point operate normal, and the general condition is good, you may want to give it a second look? If it is a one owner, that is another plus too? The tach hours may not be accurate, because of broken cables in them? I think the price is fair also?
I have used a 60" Howard J rototiller behind my 1500....20 HP/17 PTO with no problems. I also use a 60" box scrapper for grading purposes too! It is a great tractor; and it will wear me out, before I ever wear it out. Good luck on your search.