I have an '88 420SEL which I got for my daughter (75,000 miles) and proactively flushed and changed all the fluids (transmission, coolant, brakes, power steering, differential and oil -- to Mobil 1) on purchase, and plan to continue to renew at regular intervals (3,000 miles for oil, 15,000 for transmission, etc.).

I also have a '98 C43 AMG (now 44,000 miles) which gets regular 3,000 mile Mobil 1 oil changes (rather than 10,000 mile FSS intervals) and has been otherwise under regular dealer service schedules. Should I also be aggressively changing the other fluids (transmission, brake, etc.) on the newer car and, if so, at what intervals?

Comments on the fluid service levels for the '88 are also welcomed. Thanks.

The coolant is done every 3 years, so you're overdue for that if it hasn't been done yet, and the brake fluid every 2 years, so that is WAY overdue by now.
No official change for the trans fluid, it's synthetic ans considered lifetime fill. But I agree that it may be a good idea to replace it at maybe 60 to 75K miles if you intend to keep the car a long time.
Gilly