Maybe its something simple like a dipstick tube seal or a pan gasket? Rear seal likes to go as well. Would be silly to take out the powertrain just for that.

You can smoke test the transmission through the dipstick without oil in trans to see if it's something simple.

But I agree with you that taking out the powertrain would be the way to do it if it's more serious or if you just want to make the car more reliable.

Because it's an 87 I would check for camshaft wear. Timing chains dont last more than ~140k on these engines so check for wear on that too.

Head gasket brands - elring or geniune mb (I think that the genuine one is made by elring anyway)