If you're planning on lenses 120mm or longer, a newer model with the gliding mirror is nice to have. My body doesn't have it and I find myself using my 120 and 250 way more than I ever thought I would. I have to estimate my frame coverage a lot more than I like. (turns out the 553 ELX I found turned out to need more fixing than it's price warranted).

Because I work almost exclusively on a tripod and I'm not 6'6" tall, I find the 45 degree finder exceedingly useful as I don't always want my camera low enough to allow me to use the WLF. I was able to find one of the later versions, the PM-45, and haven't use my high-mag chimney finder since.

I fell in love with the Hasselblad when I was 11, I was late in my later '50s when I finally got it. I love it.

KEH is a good resource. I've been very happy with my purchases from them. Craigslist locally has been a fine resource too. (Vancouver has very active photo section)