The oil pressure switch is easily changed. Simply unplug / disconnect the wire lead on the switch. Then, loosen and unscrew / remove the oil pressure sensor.
Try this weblink for the service manuals; http://www.startekinfo.com/.../disclaimer.htm

Check the many different on-line auto parts sources; such as - www.autopartswarehouse.com, www.partsgeek.com and www.rockauto.com for the replacement parts. For example, they (Rock Auto) list the oil pressure switch from around $57.00 for your vehicle.

There is no way to assess the relative worth or value via the internet - an estimate must be based on an actual inspection - up close and personal. The value would be based on; condition and maintenance performed. You can check the VIN numbers to determine the actual model designation.
Remember the 380 designation means 3.8 L V8 engine. If it has a straight six cylinder, then there was an engine swapped at some point - and this would affect the value.

Do not understand the question - what is meant by 'prize'?

did you check the glow plug relay? when you turn on the key, does the yellow glow plug light come on for a few seconds and go off or does the light come on after the engine starts? if its coming on after the engine starts it could be a bad glow plug (yes, you can get a bad one so you should check them again) or the relay may be malfunctioning not allowing the glow plugs to get hot enough.

if a diesel then its the stop valve solonoid on the injector pump for a vehicle of this age,as for the door locks and the gear change then check the vacumn pipes from the inlet manifold as they are no doubt porous also the vacumn reserve tank as well in case it corroded through,the gearchange and the door locks all work off the vacumn of the engine ,sorry if it is a diesel then it will have a vacumn pump driven from the engine either on the end of the head driven by the cam or on the back of the alternator .look at the back of the injector pimp as some old vehicles it was taken from here

you have a vacuum leak somewhere. check all the vacuum lines.

You need to Clean intake manifold.

... and EGR valve to.

check the turbo. there is a valve on the turbo that opens by vacuum when you hit the gas to allow compressed air into the intake. if you have a vacuum leak or broken vacuum line, the valve will not open and the symptom is exactly as you described. locate the valve and follow the vacuum lines to see if you can locate a leak or disconnection. i had the same problem recently and somehow, the vacuum lines had become disconnected. i probably knocked it loose while checking the transmisison fluid and leaning into the engine compartment.

The 380SL means '3.8L' originally with a 3.8L V8 engine. If the car has a straight six cylinder, then it was replaced somewhere along the way.

I suspect that you have a bad alternator. when the modules that control the other things like abs and srs receive low voltage they report it by turning on warning lights.

Most alternators these days incorporate the voltage regulator, so I would venture a guess unless you have a way (Meter) to check whether the alternator is outputting voltage at the correct 13.5-14.5 voltsDC it would seem your alternator is the problem considering information you've given.

Replace the fuse for the overprotection relay. When this fuse is open, no power will get to the cold start injector. 10amp fuse behind battery.

Check the 'How-To-Guides' feature on the AutoMD website. You'll need to loosen (release tension) on the drive belt tensioner to install the new drive belt.
Try the many on-line auto parts sources, such as; www.autopartswarehouse.com, www.partsgeek.com and www.rockauto.com for the replacement parts.