Its been too long since I replaced one for me to give you specific info. But as I remember it was straight forward. The one I did (300SEL6.3) was next to the tank, not inside.
Putting the pump in the tank is the most hairbrained thing I ever heard of. Don't the people who do dimwitted things like putting the pump in the tank realize how much shops charge an hour and because of stupid designs take 5 times a long to fix? It the car manufactyres had to pay the repair costs, which usually are beyond the warranty, maybe they would design for easy serviceability.
When I worked for IBM, reliability and serviceability was always a design criteria. Since many of IBM's machines were rentals, it was to IBM's advantage to have the fewest breakdowns and when they did happen, the machines had to bequick, easy and economical to repair. Not so with the car companies.