I know Brembo (OEM supplier) wont sell public, I think you can get Brembo rotors via NOPI. MB DOC is the resident parts guru. I'll defer to his wisdom for the specific parts questions...
The 400E's don't suffer from any known electric governor. It should run out of muscle somewhere around 150-155 though. The 500E is electronically limited to 155 but I don't know of any specific governor on a 400E. I've seen published numbers of around 150. Given the typical MB performance "under-rating" of that era, combined with the ultra tall gearing (137mph in 3rd) 155 or perhaps even 160 is possible. I have never personally verified top speed on mine, but the car still had plenty of strength left shifting out of third at redline.... Oh, while we are on the topic of high speeds, those magnetic CB antennas let go at about 125 or so. (so I'm um...told)
I'm glad your enjoying your car...Lee