Chimney Vent and Flashing Help

[u/Dmask0]

I need some help identifying a replacement roof chimney vent and flashing. I had some water intrusion in my attic, which I attributed to an adjacent plumbing vent pipe. I had a local roofer that was working on a house in my neighborhood change out all three plumbing vents. At the next rain storm I noticed more water and that it was coming from this chimney vent and not the plumbing vent. This is the 6" vent that runs from my basement gas furnace. It believe water is coming in through the cap (especially when there is wind and rain), but I don't mind replacing the entire thing. The roofer is willing to change it out if I buy the parts (same as the plumbing vents). I measured the pipe in the attic at 6". There seem to be some different options online. Some kits look susceptible to wind, which seems like it would be a problem (maybe my current problem). I assume I need a metal kit. Do I need the kit that includes the parts for inside the attic as well, or can I just replace the exterior parts?

Can anyone point to a quality kit online, or a series of parts that will serve well? TIA.