What king of 1990 Mercedes Benz? And what king of engine?
A 1989 Mercedes SE would be a W126. The 260 SE and the 300 SE/SEL of that vintage share their engines with the W124 260 E and 300 E (both M103). So cross-fitting is a no-brainer.
Either of the M103 engines (2.6 or 3.0) will slip right in, and so will the later M104 engines. A V8 would most probably require major modifications. When Mercedes created the 500E, they had to get Porsche to rework the engine bay and transmission tunnel to accommodate the larger engine.
I'm not sure of both the M103 engines use the same control module. Perhaps someone else can elaborate on that.
Same control unit for both engines, but with different parameters for fuel to suit slight
displacement difference. MBZ made mods. to engine and trans. tunnel in house when they installed the M119 for both 400 and 500E's. Porsches collaboration was mostly due to production capacity on the 124 line.
Obviously MBZ looked to Porsche for some additional performance engineering and also wanted to bill the car as produced on a "special" line not to be confused with the everyman 124. The 400E is everything the 500E is except for the appearance of being lower with wider fenders and wheels, and obviously reduced engine output due to smaller displacement. My 400 pulls 155mph just like my 500 it just takes a little longer to get there!
If the M103 drops right in, and the price between the 2.6 and 3.0 is bearable, I would highly recommend the 3.0L engine. It's a little worse on gas, but the performance is great. Better around town, smoother on the freeway.
You would save a lot of money if you rented an engine hoist and did it yourself. If that is within your abilities. If not. It depends on the mechanic, but I can pretty sure it is going to be over $1000 at most shops, not including the engine. If you can find a good one, you might want to go after the weekend or shade tree mechanic. Just my .02 cents.
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