I am currently in the middle of replacing my front seal as I'm writing this. It doesn't seem to be too crappy of a job, but I'm not done yet and anything could happen. The rear seal involves removing the transmission but there is a great how-to posted on the
http://www.240sx.org site. Once you replace those seals, have the motor steam cleaned and then get under there yourself to source any other leaks. It might not hurt to replace the oil pan gasket.
Assuming you have all the tools, the cost should be $25 for all the seals and $50 for a steam clean. That's a hell of a lot cheaper than $1500!!!
Good luck.