It seems to take 2-3 seconds to engage when shifting from D to R or from P to D. When I shift into P and take my food off the brake the car always rolls 6-8 inches. Is this normal?

Probably OK.

MB has huge gears on their parking pawl. Which makes engagement require a significant degree of rotation. It makes park roll a piece. I doubt anything can be wrong if it does engage.

Reverse does take a while longer than many trannies, but gets worse when failing so it could indicate a problem. Don't worry about it unless it changes.