If the '84 300zx does not have T-tops it is a fairly desirable car, there were only 10,000 made w/o t-tops. The automatic can be swapped for a manual, read on www.z31.com for how to do it. Parts for the 300 are not that hard to come by as a lot of the cars are going out to pasture, a parts car can be had for nearly nothing. Besides rust they are pretty rugged. Just change the timing belt & follow the OEM red line. Look hard at the body for rust or accident damage. They are however very heavy.

Don't know much about the 240 except a friend who races one and has been through 2 motors in as many years.