Seems $900 for oil pressure switch and filter is way overpriced unless they did more work than you mentioned. Now that you suddenly need an $8000 motor is alarming. Are you the original owner, how are you on doing maint at suggested oil change intervals? If it really needs a motor which Im not convinced of I would suggest to trade it for another model just my two cents. Let's see what the other members say.

 

I agree with Welcher on everything. Seems like this whole situation is crap starting with $900 for the oil pressure switch x 2. A truck with only 115,000 mile on it should not need a new motor unless it wasn't maintained.
I am not convinced it needs a new motor either. Does it run OK otherwise? Do you know the full maintenance history of it? Does it burn oil?

I guess the first thing I would do is find a different shop to look at it. Let them know the history and ask for their opinion. And even if you need a new motor, I think $8000 is way too much. Even a new crate motor should be much less than that and a good used motor less still.

 

The PO573 has to do with cruise control so I believe that has nothing much to do with it. The PO521 on the other hand has to do with oil pressure. I agree with the above to go to another shop and have it looked at, this one seems to be money hungry. You could also hook up a mechanical gauge to it. Another shop could replace the OEM switch for much cheaper. Someone who knows what they are doing can replace the switch and screen pretty quickly (maybe an hour or a little more). Someone not familiar with the process could take a lot more time because basically the switch is hidden.
Let another shop look at it even though it might not be at a convenient location. A dealer would probably cost you more than a private mechanic. Best of luck to you.