Let me just add that the price difference between BGN and EX is not that much, if we're talking about rbs. Also note the inexpensive kit prices... good thing about a kit is that you can try everything out instantly.
I always buy bargain grade and have been happy. Generally, I just assume that the backs are going to need their seals replaced so I've learned how to do it myself. It's very easy. Now that I can do it, I buy UGLY grade backs for dirt cheap.
I purchased my M645Pro as part bargain grade, while other components of it were very-good. While the back I purchased from their eBay store started out alright, it soon developed some problems. Also the Sunpak 622 flash was DOA.
A new back, is in order, as well as a brand new 622 from B&H. Otherwise, I've been very pleased with everything I've purchased from them, starting a decade ago.
If the lens doesn't read "ZEISS", then it just isn't.
I purchased two RB67's from KEH. Dropped the first one on concrete. Both were rated bargain, and both looked and worked great. I think they cost around $140 each for the body. The guy on the phone was helpful and shipping was fast. I have used them since to purchase 6 lenses for the Mamiya and my 4x5.