Tl;DR version: I’m trying to get input on my action plan for fixing my slow 1987 300D with 255k miles (20-25 second 0-60 time). I have a tentative plan which I’d like input on.

Long version: I recently inherited a 1987 300D with 255k miles from the original owner, my uncle. It wasn’t running, so I had it towed to my house where I swapped the battery and it fired right up. It had a super rough idle that shook the entire car, so I took it to a local indie Mercedes mechanic to diagnosis. He changed the oil, replaced the injection return hoses, said he “repaired fuel injection” (as per the invoice, need to get clarification from him on what that means), and replaced the overload fuse. When I got the car back and the car idled as normal, so I put new tires on it since the old ones were dry rotted. After driving it for the first time, I found it’s extremely slow. I didn’t think anything of it at first (being the former owner of a 1989 Land Cruiser, which is basically a glorified tractor), but I’ve since seen many sources say that the car is capable of a sub 13 second 0-60 time.

I tested the 0-60 time on a slight decline, and it was in the mid 20’s. This is probably partially due to the car being reluctant to upshift until the RPM's are high (and it feels like a kick in the pants when it does, but that’s another issue for another day, thinking it might be the transmission modulator). It has decent off the line to 10-15 mph (seat of my pants observation), but no guts after that. I’ve only had naturally aspirated cars, so I’m not absolutely sure, but I don’t think the turbo is spooling, or at least I can’t tell any difference. After a few hours of internet research, I came up with a tentative plan. Before I dive in, I’d love to utilize the collective wisdom of this board to make sure I'm on the right track, and hopefully provide a plan of action for others who might need direction.

Current Plan (In Order of Escalation):
-Put fresh diesel in tank (current fuel is a few years old)
-replace fuel filters
-Open air intake; replace air filter if necessary; make sure it’s clear all the way to the turbo. Check to make sure airflow sensor is clear
-Remove the line going from the intake manifold to the switchover valve; clean nipple, ensure hose isn’t cracked and is clear
---repeat for the line going from the switchover valve to the ALDA
-run line from intake manifold directly to ALDA; if performance improves, replace switchover valve (haven’t been able to find a part number for this yet)
-remove ALDA; if performance improves or black smoke shows upon acceleration, reinstall and adjust via shims or screw on top
-check wastegate on turbo to make sure it works. If not, replace with aftermarket wastegate for 602 from atpturbo (haven’t been able to find a 603 wastegate)
-make sure turbo spins freely, doesn’t have excess play. If it does, service or replace
-remove/cap EGR (I’ll eventually do this regardless of whether or not the issue is fixed by this step, as my car is exempt from emissions)
-drop fuel tank, clean screen
-cut out catalytic converter (or whatever it is that Mercedes put in when they replaced the trap) in case it’s blocked. Replace with straight pipe.
-rebuild/replace injection pump
-take car out back behind the shed; put it out of its misery

Do you think I’m missing any steps, have any errors, or should rearrange the order? I’m going to start at the beginning tonight. Any input and/or feedback is appreciated, and I’ll update the thread with my progress to hopefully help others with this issue.