I am in the middle on an S54 swap on my E28 at the moment and would like to add some things that Charlie hasn't or maybe is new info since his build.
-E28Goodies mounts suck but are 1 of 3 vendors I have seen that produce engine arms for our cars. Another vendor "Classic Daily" produces a set that are much bulkier, beefier, and don't suit the engine in the car any better. The issue is that these mounts are produced and made for the S50/S52 or M50/M52 engines. The S54 has a taller profile due to the plastic beauty valve cover and the fuel rail. Because I had already paid for the e28goodies engine mounts and couldn't return them, I decided to modify them. Essentially I just remade the flat plate against the block and lowered the engine down on the mounts 17mm. This is adequate to close the hood as well as create a flatter engine profile instead of it leaning in the back towards the firewall.
-I am using the Getrag 420 6 speed E46 M3 transmission as well as Charlie. Some things to consider if you chose this over the ZF 5 speed: firewall clearance, transmission brace, mounts, driveshaft, and shifter. You will need to clearance(BFH) the firewall right where it goes from being flat to turning down into the trans tunnel. You will see factory spot welds there where it laps into the floor board of the car. Hammer it in about 150mm in length and about 10mm depth. It is completely okay to clearance the firewall in this area because its under the HVAC box and the carpet has a lot of space below the box as well so it will not interfere. For the trans brace you can just buy this from fellow member. He doesn't have the S54/420G listed but it fits the application.
https://www.limitedslip.net/product/e24 ... egory_id=4. For the mounts I just used 034 Motorsport E46 M3 rubber reinforced mounts for street like NVH. In regards to the driveshaft, you can fiddle around and try to make something or have someone build you a driveshaft but I have found that a 1983 533i automatic driveshaft is a bolt in direct fit driveshaft. It uses part number 26111209508 or Dorman 936-394. I purchased a brand new Dorman unit as they are made in the USA with heavier duty u-joints. I am using a modified E46 M3 shifter with a fabricated selector rod and DSSR. If your car is automatic, you can use an E46 M3 shifter floor bracket and riv-nut it to the floor instead of cutting and welding a used one from a donor car.
-I am using a karbonius carbon CSL airbox with my build. This airbox as designed to be in the E30 so it has a cavity that is built into the airbox to clear the brake booster. Most folks doing this swap can keep the original Hydroboost brakes or convert to vacuum. I opted to use a RHD E90 M3 brake booster because its small in diameter but thicker so it has the brake assist as well as an updated diaphragm as opposed to a Porsche 944 booster. I also moved the booster to driverside 0.5' over and 0.5' down. I mimicked that on the pedal box as well so holes lined up. You will have to modify the clevis as well.