Cap
Rotor
when was the last time they've been changed? i ask this, because it definitely sounds like heat increases resistance in the ignition system and you won't start with weak spark type of deal. another possibility is the crank sensor on the bellhousing. eventually they fail and you will NOT start period. without the crank sensor, you have no spark. pull the cap and rotor and check for corrosion. there is sealing o ring that seals the cap and rotor internals from moisture and this oring hardens and cracks over time with all the heat it is exposed to. when it's cracked, you'll have VERY bad corrosion on the terminals.
why have you narrowed it down to fuel injection? sure the accumulator could be a problem, but your car WILL start with a bad one, same for the check valve. you car doesn't start period, right? if the accum. and valve were bad the car will catch and stall a few times before
eventually starting. you have to realize, it's HARD to diagnose with just words, you'll have a lot of ideas thrown at you with this problem, but start with the easiest and take a plan of action if something doesn't work out.
also, 4 seconds of cranking for a CIS M104 is considered NORMAL by MB. it may help applying 1/4 throttle at start up if cranking for a while.
i've went through the ENTIRE ignition and fuel injection system on my own 92 coupe. it was hell, lol. PREPARE!
