What you are facing in your range of choices is the line between OBD I and OBD II. What you really have is a 4 cylinder and a 6 cylinder. The 4 comes as the M42 and M44, the 6 comes as the M50 and M52. There is also a car called an M (for Motorsport), but that engine is the S50 or S52. In all of these choices, the OBD I cars (before 1996) is the first listed, and the OBD II cars (1996 and after) are the second listed.
You appear to be only interested in the M52. This motor can be had as the
323 or 328. The 323 has a 2.5L displacement, the 328 displaces 2.8L, and both of these motors are essentially the same from a mechanical perspective(a difference such as this can be accomplished by changing the crank shaft and connecting rods).
There is not really a dizzying array of selections.
PS OBD (On Board Diagnostics), I and II are the engine management schemes. OBD II is the better choice, but it comes into play from the 1996 model year, so your search strattles the line -- and hence the extra confusion. As you move into the '99 and later models, the 325 reappears along with the 323, 328, and the addition of the 330.
Shop for your price range, it's far easier than shopping for engines. As a practical matter, most can't tell the diffference in the engines, especially on the daily grind to the office and back.
So, Jeffy, we've seen the end of you dividing 328 by 325 in the midst of a discussion of engine torque? I recall that you concluded that "the difference was less than 1 percent" LOL
(Yes, the above really happened. Jeffy's not the brightest bulb on the tree.)
Perhaps so, However, the model which was called a 323 actually had a 2.5 liter engine. I can think of two reasons:
The 323 did not have VANOS on both intake and exhaust valves. The 2.5 liter engine actually had the performance of a 2.3 liter version of the 2.8 engine.
Buyers of the 328 would be upset if they saw how much cheaper the 323 had despite have only 300 cc less displacement.
You should not read too much into those labels. The sheet metal just sits there and lets them put whatever label they wish.
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