Bronicas are good to start off with but be warned: the shutter mechanism is super strong on some models (especially 6x4.5) so much so that it makes the camera jump a little when it's released, the lenses are a bit soft and I've experienced them to break down a lot. They are cheap to buy though, you can get a standard kit for around £350.
Hasselblads are beautiful cameras, you can't get better for medium format - the optical quality of the lenses are incredible and they're very solidly made. the downside is they're very pricey. you could maybe pick up a 80mm/body/waist level viewfinder/120 back kit for around £800 or so, but lenses are upwards of £400 each and are often over £1000, so buying one is a huge investment.
Mamiya are another great manufacturer to go with. The Mamiya C330 (6x6) gave me the sharpest images I've ever seen, the detail just pops, even when looking at the negative! problem is that it's a twin lens and you might not want that... so as far as SLRs go: they do a 645
(samples) - which is really comfortable, and easy to hand hold or a 6x7 camera which is a bit of a beast. Good prices, good quality, slightly cheaper than most without much compromise.
If you really crave a great quality square format medium format SLR, as close to a Hasselblad as you can get, minus the excessive premium... try to find either the Rollei 6001, 6002 or 6003.They can take Hasselblad lenses if you ever want to upgrade, but they also take Schneider and a couple of other types that yield really great results. Check out:
some samples. Or a bit about each
here. then again, those lenses can be very pricey too...
Personally, Mamiya seems like the best mid-way option. If you know you never want to upgrade and you're not going to be heavily scrutinising the sharpness of your images, go for the Bronica - it's the least expensive, and can handle more of a beating because the parts/accessories are much, much cheaper to replace...
Hope that's of some help. I've had personal experience with most of these.