The M103 and M104 blocks only have detail differences that won't affect you if you use it as a complete long block. The oil pan won't interchange between the two due to the M104 using a double row timing chain. There are 2 M104 oil pans so select the one with a sump position that matches your car.

The best year in terms of FI system will be a Bosch ME 2.0 / ME 2.1 ( 97 C280 / 97 E320 / 97 SL320 / 97 -99 S320 ) as this has drive by wire not the troublesome half cable half servo throttle.

97 + also gets you the electronic shift 722.6 To make this work you will need the ABS computer and feed it with the proper road speed pulse. This can be done by using a electronic speedo fooler to get the ratio the trans computer is expecting.