773 was a very good midsized machine. Vertical lift, not too big or too small. Probably one of the most popular sizes. Overall very reliable machine. 1700 hours is not too bad but depends on condition, options etc. $12k price is also good price if in good condition. Those I have seen some at auction will bring $12-15K but with soft construction market, that maybe changing. Probably still has a lot of life remaining. Anything getting near 2500-3000 hours is probably getting beyond anything I would consider.
I bought my 1998 873 F model with cab heat, hand controls, and bucket at 1200 hours in 2003 used for $15k. That was a good price. Bought from a Cat dealer who wanted to get rid of it. Traded in last fall for 2006 S300 with 700 hours. Got trade in of 18K on 873 with 1800 hours but still in good shape. Dealer resold it about 3 month later.
Is it dealer or private sale? Does it have a kubota motor? What do you plan to do with it? How many hours/year will you add?
The boom design does create some blind spots, but there are certainly worse machines for visibility with vertical lift. I didn't have major visibiltiy issues with the 873 which used similar design. Visibility out the back is overall average among SS.