89 rs / 305 bad computer? 66,000 miles

all of my sensors quit working. temp. gauge doesn't work and the oil pressure gauge will kick on after a few minutes. engine light is on. codes that are stored are for the oxygen sensor, temp sensor, and the mapp sensor. i bought this car a few months back and the owner said it has sat for years, which i can tell. after digging into this car and can tell this isn't the stock 305 in it. can i jump the temp. sensor to see if i get a reading or will this short it out. is there a way i can check to see if the computer is ok.
and know i found out the alternator is bad.
any ideas? what would a used computer costs?
thanks for any help...

It's highly unlikely that the computer is bad. Check the fuse panel for the gauges fuse. If it's blown, just try a new fuse. If that goes or even if it's good, start looking for a cracked wire or two under the dash.

For the other sensors that are throwing codes - disconnect and reconnect them at the sensors a few times, and disconnect and reconnect the computer a few times (WITH BATTERY DISCONNECTED), to see if the connectors were just a little too corroded. Replace the O2 sensor anyway - with an AC Delco sensor, not Bosch. The coolant temperature sensor typically has a long life, so it's probably just the connection, or a bad wire somewhere.

The MAP sensor is the main sensor for determining the fueling. First check the connection and wires, and the hose that connects it to the intake manifold. If those all check out fine, replace the sensor.