For me the three were:
1. f/3.5 is enough
I even gave this advice for a friend, he ended up buying two 2.8 Flex at different time, and I ended up having the older of his two
I still have my Cord III with 3.5 Xenar but I'd really like a 3.5 Planar.
2. You don't need that RB kit
This RB kit that came with two backs and the 50/90/250 lenses. Only shot like 4-6 rolls thru it in more than 2 years, never go out with me anywhere because practicality of a TLR wins me over. When it does came out though, the negatives are so so good and makes it worth the hassle.
3. Get a hood, a Rolleinar, and a good CLA
Sorry I cheated on this last bit but I think just this point will make me a happy MF shooter.
P.S. This thread is loaded with great advices I'll keep in mind.