There's only about 10+ bhp difference between a 2.0 /4 and a 2.0 /6. The /4 has more low end torque and the /6 also weighs more than the /4. So there is a difference, but not much and it pretty much compensates and evens out.
The OG /6s are too overpriced now in most cases. If you're looking for a car to drive and enjoy, I don't think the cost difference for such little difference between the 2.0 /4 and 2.0 /6 is justifiable. It's more of a collector's car now. For a fraction of the price, you can get a nice 2.0 /4. You can also easily get get more power out of the /4 also. The simplest way is to do 96 mm bore and pistons (stock is 94 mm) and larger jugs. That will bring it up about 66 cc. There's all kinds of way to get lots more power out of it.
Also, ditto on the advice on the odometer. There's no way to know the actual miles of a car. The odometer can be turned back (mine was when I had my gauges restored). My previous gauges were not period correct anyway which is another thing. The gauges can be easily swapped out. So there's no way to tell.
Find the nicest example you can. Matching #s is really only for the uber collector who isn't going to drive the car but just sell it anyway to another collector. It's just a number stamped on the case. Hell you can change the case, change the insides, once you rebuild it and replace everything inside it's no longer "original" anyway. If you want original, just make sure it's a good, working, year and size correct unmodified engine. Have it run the way it did when it came out of the dealer. If you're a stickler for a number stamped on the engine tin, then go for it. There's always that appeal of being as original as it can possibly be. Once a car's been restored, it's no longer original. But you can keep all the parts original and have it to original spec. If you change out a gear for the same gear, it's not the original one but it is the same one and drives the same way.
Anyway, best of luck on your search! I hope you find exactly what you're looking for! Happy motoring!