Buy OEM stuff (Mercruiser or Quicksilver brand), it's quite good and reasonably-priced. Buy an entire kit and new gear oil, don't re-use any of the wear parts. I think the kit is part number 817275Q05. It will last for years.
I'm not a fan of Sierra rubber parts.
Not much to check when you split the Alpha drive open, but you should check engine alignment, gimbal bearing, u-joints, bellows, and shift cable, and make sure the gimbal ring clamp bolts are torqued properly.
If the drive is only used in fresh water, you can replace the OEM aluminum alloy anodes with magnesium and reduce corrosion dramatically. After-market magnesium anodes are OK. This is a really good idea if you have a stainless steel prop.