Your missing the "nickel and dime" factor. These things will nickel and dime you to death. A rebuild with shipping is 1200 alone, if you did it yourself you could probably do it cheaper but I dont know how much cause I have never done it.
It says the motor is not complete, so whatever it is missing you need to find, I am going to guess that he started parting it out and the rave valves, head, magneto are probably gone. So you will need to find whatever is missing or pick up a core somewhere. $3-500
Then there is all the little stuff it might need oil filter, gas filter, rebuild carbs, clean raves, alignment tool if you don't have one, seat, throttle cable, possible pump rebuild, impeller, wear ring, etc
After that then you need to consider the cosmetic items you want to do.
I am not saying that all the above needs to be done but there is that possibility. IMO unless you have all the tools and know how to get everything done (including the rebuild) in your own garage, and are able to get good deals on all the parts, you will not get this on the water as a presentable, reliable ski for 1500.