are they good cars to buy or do they break down alot

l assume u mean 90k miles. Cost will be dependent on the mods if any. On the 240x look to drop about 3500 unless its has rb20 swap. Then look to drop about 6. On the 300zx hopefulling its the z32 (90 -96) depending on if its the TT (twin turbo) or N/A (non turbo). The turbo in really good condition look to drop btwn 5k - 13k. Non Turbo from 3k - 7k. Both r really good running vehicles, but the 300zx can be very costly to repair. Make sure the 60k maintance package was done before u purchase it & ask for any reciepts of any work claimed to be done. If the seller cannot produce records or reciepts walk away. The 60k maint package includes, replacing timing belt, water pump, thermostat, inlet & outlet water hoses. Its the most expensive engine maint package. Also check for signs of high pressure power steering hose leaks. This leaks builds up on the alternator & is easy to detect visually. Just do some research on these cars. Alot of people want to be home tuners & do not know what the hell they r doing & screwing both vehicles up then selling them.
Good luck