Aftermarket vs OEM replacement parts

Hi all,

I have a 2004 350Z Enthusiast that recently started having some mechanical issues. I replaced the ignition coil, spark plugs, Knock Sensor, steering rack, and front tie ends.

The mechanic also noticed an oil leak from the right variable timing valve which is leaking into my alternator. I also need to have the front control arms and lower ball joints replaced, along with the rear shocks. As this is all adding up to be quite costly, I've looked on Ebay for some of these parts.

I've found some of the aftermarket items seem to be WAY cheaper in cost than OEM (example, there's a pair of timing valves for ~$40 vs one OEM for ~$180). I want to make sure I'm getting good quality items, but I also don't necessarily want to spend top dollar replacing these parts.

I'm curious if you all have any suggestions or recommendations based on your experiences? Happy to link the eBay items I found as well.