The part you saw on Fastlane is the primer pump only, which is the part you need if yours is leaking. It's simply a matter of unthreading the old one and refitting the new pump. This will fix the o-ring leaks you are experiencing now. Less than a half hour and you'll be leak free with a new pump primer. If you order the other part it is the lower half or the actual fuel pump. Probably not what you need as described on your post.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan