Wow sorry to hear you had so many issues with light leaks. I found mine at Adorama with the Ai AE prism, the motor grip, and a 120 back all in great shape for pretty cheap. I bought some more backs from KEH and Roberts Camera on eBay, mostly Ai but one SQ-A in varying conditions and none of them had problems either. I guess I just got lucky.
I too considered a Hasselblad (and honestly still do) but curiously all my friends that owned one were less than enthusiastic in their recommendations based on long-term reliability and repair costs. Some of them had theirs lock up on shoots and required an expensive rushed repair more than once which was surprising.