If the first car is indeed as you describe, it is considerably modified. An 84 760 GLE originally had a B28F V6. There was also a 760 Turbo that year, which does have a B23FT (not an E, those were never sold in the U.S. market) but it is not crank fired. There were no 740s until the 1985 model year. All 4 cylinder 7 series cars had a Hall effect distributor, mounted on the back end of the cam, until 1989, which was the first year for the crank position sensor. Turbos made do with the old setup for a year longer---1990 was the first year for the CPS on those. Cars with CPS still have a distributor, in the same place as the earlier models, but it is an empty shell, with only a cap and rotor, which are the same on all years with Bosch ignition. If the distributor is mounted on the left side of the engine block, toward the front, it is a 240 Turbo engine, which could be either a B21FT or B23FT, depending on the year it is from.

Fixit is correct about the wiring harness being prone to dterioration, but if the engine has been changed, the harness will have required considerable modification. Depending on how that was done, it could have cured most of the potential problems, or be an absolute nightmare.

Note also that many detail improvements took place in the 7 series cars over the years, and as a result, the 91 is a much better car than the 84, particularly in things like the durability of interior trim and power window mechanisms. 91 also has air bag and ABS.

740s are one of the least rust prone cars I've ever seen, so I would not rule out the 91 because of where it it from. I would, however, inspect it closely underneath. As long as it wasn't driven on the beach or used to pull a boat trailer out of salt water, you are likely to be pleasantly surprised by its condition.