There are a few special tools needed when doing the valve clearance check.
The most important one is the BMW valve shim remover/installer, which has a magnet that helps prevent the user from dropping a shim into the crankcase via one of the oil galleys or into a cylinder via a spark plug hole (if you're foolish enough to not plug those holes). This tool is BMW 83.30.0.493.743, and was previously BMW 90.88.6.114.400.
On some S54 engines, a special tool is needed to turn the engine via the crankshaft. It fits over the bolts on the vibration damper. This tool is BMW 83.30.0.491.056, and was previously 90.88.6.115.100.
Feeler gauges are also needed. The specification for valve clearance is .18mm to .23mm for the intake, and .28mm to .33mm for the exhaust. I've not found metric feeler gauges with these exact sizes, but .18mm is .007", .23mm is .009", .28mm is .011" and .33mm is .013". The inch sizes are easy to find.
There are really 2 different uses for the feeler gauges. The first is to measure the existing clearance, for which I could use better resolution than what's available in off-the-shelf feeler gauges. The second is to check the clearance when a new shim has been installed, and step-ground ("go/no-go") feeler gauges are near ideal for this task if the range is correct. I bought some Eastern Industries step-ground feeler gauges (FG-22-STEP), but the blades are straight and somewhat short. I will probably bend the ones of interest. I also bought the Eastern Industries FG-26-4-45 set, which have 4" blades with a 45-degree bend but are not step-ground. And since my older sets of feeler gauges have seen better days, I also bought Eastern Industries FG-25-3-SS which are stainless steel flat feeler gauges with 3" blades.
I requested a quote for custom feeler gauges, but have not received a response. I'll try again soon.
A micrometer is needed for measuring valve shim thickness. I use my Mitutoyo digital 0-1" micrometer.
A slew of gaskets are needed:
There are a number of places where you can buy the shim set and the gaskets as a kit. I bought mine from ECS Tuning for $445.00, and it also included the valve shim remover/installer mentioned above. Turner Motorsports has the same kit for $499.