In 1996, the only model was the E320.
(Or the E300D)
The E420 has the 119 motor, which was a unique motor (as compared to the M112/M113, which I will explain).
The 119 was used in a few different chassis and a couple different displacements. It is a dual overhead cam, single plug per cylinder, 4 valve per cylinder, and earlier versions were CIS-E, like what was used on the M117 (560SEL) and 103 (like the 124 chassis 300E for example) and later they had them with different versions of SFI (Sequential Fuel Injection). It was a very tough motor, capable of putting out ALOT more horsepower than Mercedes actually tuned it to.
The M119 (I informally refer to it simply as the 119), was found, in the US version vehicles, first in the 1990 500SL (later it was called the SL500) up until I believe 1999, after that it was still called the same thing, but the newer M113 motor). It was also found in all V8 140 Chassis S Class, (400SE, S420, 500SEL, S500, etc), it was also the only V8 put into the 124 chassis, such as the 400E and 500E.
And of course out subject car, the 1997 E420 also used the 119 motor. Here is a link to motor specs on the 97 E420:
Now in 1999 they started using what is loosely referred to as the "Modular" engines. What this means is that they used similar technology and even some common parts to make up either a V6 (Mercedes first) or a V8. The V6 differed mainly in the amount of cylinders (obviously) and a counter balance shaft. Some of the parts you can see shared between the V6 (M112) and V8 (M113) are things like pistons and rings, connecting rods, wrist pins, valves, valve springs, some of the timing parts, fuel injectors, some emission equipment, etc.
The M112 engine was first found in the US on the 1998 ML320 (It came out quite a bit before the cars did, although same model year). The 3.2 liter version of the M112 was also found in the E320 and CLK320. they never put it in the 140 chassis or the 129 (SL) chassis. They did eventually put it in the 170 (SLK). They also built a 2.8 version of it for in the 202 chassis (C280 in 1998).
The M113 V8 came out in 1999. Again, never put into the 140 chassis. The 140 chassis retained either the M104 inline 6, or the M119 V8. This is I'm sure mostly due to that they knew they were replacing the 140 chassis is 2000 anyways, so why bother engineering the motor change? They did however drop it into the 129 chassis SL500, with no change in badging. Most everything else that the M113 would fit into got it though, in either 4.3 liter or 5.0 liter. First few examples were the ML430 and the E430. The 500SL was joined by the new S Class (220 chassis), which you could get in 2000 in either 4.3 or 5 liter sizes (S430, S500).
They also managed to cram the 4.3 into the 208 chassis (CLK430). The M113 is also the basis for the V8's used in the AMG versions such as the ML55AMG and E55AMG.
Here is a link to the engine in the 98 ML320:
Here is a link to the engine in the E430, just to compare the E420 to:
I prefer the feel of the older M119 a little better, it has a narrower torque band, but I kind of "like" finding the peak torque.
The M113 has a flatter torque curve, it's a nice runner though.