Hi Paul,

When you start doing searches on "ASR" and "limp mode", you'll might be a little overwhelmed. First thing tomorrow is to go to Radio Shack and buy parts to make Arthur's Code Reader. You'll need at least 9" for each wire color. 10 or 12 ga. is good. I mounted the switch and light in a 35 mm film container, but any pill container about 1 x 2" or larger will work. The black box [2w x 3L x 1d"] you plug into is in front of battery towards engine.

Read the codes a few times to make sure you're doing it correctly and post your results here.

Easy things to check first:
Vacuum tubes / hoses.
# The hard plastic ones can be replaced with standard rubber vac hoses. OEM was color coded, yours may differ i replaced. But keep some of the hard plastic to make connectors for rubber hoses into elbows.
1] Behind brake master cylinder booster: feeds engine & tranny !!!
2] Under black plastic front engine cover
3] Air pump and others you can worry about later [Door locks & Climate Control]

Your questions:

OVP aka Over Protection Relay has blue fuse on top. Name implies what it does. Easy diy.

Head Gasket: Usually leaks at corner closest to passenger. Open oil fill cap, if just oil you're not mixing oil & water. If grayish or yellowish moose, you're mixing = ng.

Wiring harness is problematic the more you move it. Hopefully, with 145k it was replaced. Easy place to check is wiring at thermostat housing. An electrical connector has a black sheathing over two wires. Either slide back or carefully cut the sheathing to reveal the wires. If the insulation is dry or brittle it's ng. If smooth and pliably it's ok. OEM harness was made with a "green friendly" insulation that unfortunately couldn't withstand heat. [MB still doesn't admit they screwed up].

Throttle Body aka Actuator [see pik in my profile photos] located between 3rd & 4th intake tubes, fed by air intake tube. $450 to rebuild mine, easy diy. Uses same ng wire insulation as engine harness.

Oracle is correct about checking neutral safety switch. It's located on bottom side of tranny. A PIA, but still a diyer.

Use 'fastlane" on this site to get idea of parts piks & cost.