Hi Mat,
Welcome to the forum...and potential Contax 645 club. I probably use this system more than any other due to its tremendous flexibility. I can second everything that has already been said above. However, I should also point out that the 45-90 zoom lens is a real winner. The 45/2.8 has a nicer bokeh quality and a better choice for some types of portrait work, but the 45-90 zoom is an ideal all-around lens, and is probably the one lens that is on my Contax 645 the most.
At the wide end of the zoom, the 45mm length is higher resolving than the 45/2.8 and better for landscape. At around 70mm, there is almost zero distortion, incredible sharpness, color and even illumination. At the long end, the 90mm can shoot at 1:4 macro mode. There is no loss of image quality with this zoom lens as compared to the primes. The only trade-off is speed versus the flexibility of not having to swap lenses between the 45, 55 and 80.
Good luck in your shooting! Right now, the economics of shooting with such a versatile system may outweigh many of the benefits of much more expensive systems. Unless you have a specific need (like for professional architecture work), the Contax 645 is a great low-cost MF solution.
The cost of the zoom is a bit higher, but it replaces several lenses and is about convenience. The zoom would be my recommendation for the best all around lens for you, if the price is affordable. Otherwise, the 45/2.8 is the easiest lens to find, and is the lowest cost of the wide lenses, and is a good choice if you want to save some money compared to the 55. The 45/2.8 is second only to the 80/2 in its ability to auto-focus in low light with this system. I have seen some amazing portraits from people like Mark Tucker with this lens and camera system. You can also check out the work of James Russel who also shoots with the Contax 645 system, using a Phase One back, I believe.
I almost forgot....Marc already mentioned the use of the Hasselblad lenses. For landscape, there are many such lenses that you could use which are high resolving and can be put on the Contax 645 with one of Son's adapters.
David