There's nothing wrong with a stock (used) retainer if you can find one. The Viton rear main and silicone side seals are about as fancy-pants as you can get. I installed them on my last build. There's a trick to installing the side seals - you just have to follow the directions.
*Shoots best real main seal* Yep, I see a leak. Get a good quality one at your local auto store, all of em' leak at one point or another, especially those old 2 pc seals.