Not a bad looking ride. Sometimes you just luck up and find deals like this.
The oil pressure sending unit is probably the culprit. First, put a mechanical gauge on it and check pressure at operating temperature idle. Then replace the sending unit with a new one. Bring it up to operating temperature and check the gauge inside the car for comparison. The units available today are very often inaccurate. The BWD brand is the one I've had most problems with. NAPA or AC/Delco is a little more expensive but seem to be closer to correct, right out of the box.
The "Service Engine Soon 10 Minutes After Reaching Highway Speed" syndrome is caused by a leak in the EGR circuit. Remove the line from the EGR valve and with your fingers press up from underneath the valve, against the diaphragm. Cap the vacuum port with your finger, ease off on the pressure on the diaphragm and see if it gradually drops down. If so, you have a hole in the diaphragm. Otherwise it could be a cracked line going back to the EGR, or the little looped line on the EGR solenoid. If it turns out you need the valve or solenoid, I have a couple of good used ones.
Exhaust leak that you can't find is often the forward manifold where the number 6 port joins the manifold runner. Notorious for cracking there. Simple fix is just to drop the rear cradle, remove the manifold and have someone weld it.
The high idle problem sounds like it would be caused by a crack in the EGR tube