If it has a K plus four-digit serial number, buy it now and then come back here immediately with pics to discuss its specific characteristics. If it has a K plus five-digit serial number whose first digit is a 2 or 3, you probably ought to buy it now just because it will be one of the earliest commercially distributed K-38s, and there are collectors who will pay a premium for early production guns in a specific model line.
A K plus five-digit serial with an initial 4 or 5 would probably have shipped later in 1948 (or even early 1949) and would be less likely to have an earliness premium attached. As you describe the condition, $699 sounds a little heavy for the gun if it has one of these higher serial numbers. If you can get for a hundred less, do so.
Or if you just want the gun and would hate to let a chance to get it slip away, pay the asking price and consider whatever excess that price might include as "ownership insurance."