Picked up a 1976 240D to put my 220 motor in but wondered if the 240 motor is worth fixing instead. It's got a rod knock, not major but it's there.

I know finding engine parts for my 1975 230.4 has been a nightmare. I wanted to see if this would be any different considering I'll end up having to go oversize on the rod bearings.

Anyone been down this road and are the parts still around?

Sane question. I would have a look and see if the crank journal is damaged for starters. Also might be important to determine if it is the number one rod in question.
I have been wondering for awhile if caught early enough and everything checks out including a plastigauge check of the number two rod bearing being within specs.
Just changing out the number one rod bearing might keep the engine alive. Much depends on if the number one crank journal being still good. You do not even have to pull the engine to do it. Worth a quick check in my opinion.
A large proportion of the 616 failure is the number one rod bearing for some unknown reason. If the crank is damaged depending on where you live the availability of good used reasonable cost engines or donar cars will be a factor as well.
I have even been toying with the ideal of changing out the the number one rod bearings on my 240ds as a preventative maintenance thing or at least plastigauging them every ten thousand miles or so. There should be indications of them starting to get bad well before damage is done. I guess much depends on why they fail though.

So #1 is the only rod you can access without pulling the motor?

Maybe #2 but fortunatly it is usually #I but not always. Since I assume you do not win the lotto every week . Just hope luck or good coincidence prevails here for you. This problem can worsen very quickly by the way so really restrict usage. By the time it was established it indeed was a rod it was through the block on a couple of site examples. The last episode not long ago was #2 . Almost all the previous examples where number one.

This guy picked it up early on. Like I said, I can barely hear it but he said it's there and the oil pressure drops off when it warms up. I'm taking his word for it at this point.

Once I get it home I'll pull the pan and take a look.

Um, I'm a little confused, as I recently got rid of a well-running '76 240D and I had adjusted the valves on. My engine had overhead cam, so I'm wondering how there's a "rod" knock, if the engine is OEM.

FYI - I needlessly replaced a motor on my '84 300d due to "rod knock" - long story but my mechanic who did the swap had a friend who was a diesel mechanic assist with the operation. As his payment for assisting, he kept the old "defective" motor that turns out only suffered from injector nailing - a far cheaper fix than the diagnosis I was duped with. I now wander the streets chanting like a savant..... "injector nailing? injector nailing?" In case you haven't already tried maybe I can save you a few thousand dollars just by cracking each injector line one by one to see if the noise goes away.

Rod knock reffers to the rod that connects the piston to the crankshaft. I assume you were thinking pushrod that goes from the cam in the block to the rocker arm. These rods of course are present when the cam is down in the block.
It is not a lot of effort to remove the smaller lower portion of the oil pan to check the bearings for #1 and #2 connecting rods.
As I mentioned earlier there seems to be a tendency for the failure to usually be the the number one rod bearing. Like most things in life nothing is written in stone though. It has been proven too many times now that a suspected knock on a 240d should be looked into sooner than later.
It always pays dividends to personally know a really competent mechanic at least to assist with those problems that are somewhat subjective in nature. Their instinctive grasp comes from years of hands on experience.
I know for a fact since I have not put in the years of experience I will defer to their knowledge level every time. First I make sure they are the real thing though. A guy has to know his limits.
When one of us members is describing a sound we try hard yet it is still a second hand description so to speak. We may percieve it as something else. Or miss something of importance in the description.
Nothing replaces hearing it in person as their are so many possible interpretations of what we hear. Many may not be transferable to the written word or just are very hard to describe.