In my experience the 700 is real workhorse. I used one for years in the BVI charter trade. When it wnt a little fuzzy it went into Cay electronics in Tortola for a new board or two. Good enough for a few more years until it went fuzzy again. They told me that due to extremes in the environment the old horse was best put out to pasture.
The 710''s were the next generation. These have been replaced by their newest version the 802''s.
Bottom line, the 700''s were great. The years may have caught up with the unit your looking at. I''d have it checked out. If it checks out ok at 300 bucks jump on it.
But that''s just my opinion.