Rather than tell you what to get, I'll tell you what I've got, what I don't have, what I wish I had or plan to get, and why.
I've been doing diverse commercial work myself. Been at it for 6 or 7 years, so have gone through a lot of the same angst, but with a film-to-digital conversion thrown in to boot. I've gradually added equipment, as needs and opportunity arose. I started with two cameras, four lenses, two flashes and some other key accessories. But, I had a couple older manual focus, film systems and accessories to fall back on, too.
It's made trickier, trying to cover different types of situations, but there are items you can use in more than one application. Personally I try to avoid weddings, but have been talked into enough to know that there are special requirements if you are going to do that sort of work.
#1 priority, you need redundancy to shoot weddings. You need at least one backup camera, maybe two. The second backup might even be a film camera. (I have two 30D, one 10D, and two now-rarely-used EOS3s).
You also should have multiple flashes, two at the very least. I use three 550EX, primarily. For backup I've got a couple old Sunpak 444Ds, but more modern TTL Sunpak, Sigma or Vivitar might be a better choice. Third party flashes might also be an alternative to help your flash budget go a little further. I haven't worked with any of the modern TTL-capable 3rd party flashes, though, so get some more opinions if you consider them an option. For more formal portraits (wedding & otherwise) I have a kit of five portable Norman strobes that work well for me, along with travel case, stands, umbrellas, two flash meters, etc., etc.
Now, personally I prefer to work with prime lenses, rather than zooms. But, they just aren't practical for many events/sports and some wedding work. For those situations I use 24-70/2.8 and 70-200/2.8 IS on two bodies, probably 90% of the time in the case of sports/events. The 24-70 is particularly useful for wedding work. The 70-200 can be a bit intimidating for portraits, and is hard to use in candid situations. And, at the end of a 12 hour day you really know you've been lugging it around!
My only other zoom is an older 17-35/2.8 for occasional wide angle needs. It's always handy in my bag when I'm working with the two above zooms, just in case. I've been seriously considering replacing it with a Tokina 12-24/4. That lens is near the top of my "wants" list now: Wide enough for me, fast enough for the focal length, good reviews, and a reasonable price. Oh, and I also have a 1.4X in the bag, to pop onto the 70-200 in case I need more reach quickly.
Now working 99.9% of the time with 1.6X crop D-SLRs, my core prime lens kit is 20/2.8 (wide environmental portraits, larger group shots, exteriors, etc.), 28/1.8 (head-to-toe portraits, small groups, it's my "normal" lens), 50/1.4 (short portrait) and 85/1.8 (long portrait, candids).
Like you, the 135/2 has caught my eye and everything I've read and heard about it has been very encouraging. However, I've decided I'll just wait until Canon sees fit to add IS to it. Since it "acts like" a 200+mm on a 1.6X camera, it would be very helpful to have IS on it, IMHO. (C'mon Canon, are you listening?).
In the meantime, the 70-200/2.8 covers that focal length pretty darned well for me. I'm pretty sure that if I were using a full frame D-SLR, I'd be more inclined to get the 135/2 now, even without IS. And, if I were shooting more weddings, I'd be a lot more inclined to get a full frame D-SLR right away. Cause and effect!
Out of ten primes in my Canon kit, I think the four I've mentioned are the most applicable to your situations. The others are more specialized lenses: macro, tilt-shift and super teles. I won't bore you with the details.
Since your are doing this as a business, think in terms of added value and return on your investment. What adds value to you as a photographer providing these services, and distinguishes you from other photographers vying for the same job? Where are you going to get the most bang for your buck, investing in equipment to do the work?
Start there. Also consider renting some of the stuff to give it a test drive and better see how it fits into your work. Many places offer Canon equipment for rent. Dream lenses will always be available in the future!
Me? I'm saving up for a 200/2 IS (Hallelujah! Canon finally listened to me!).
Hope this is helpful! Let us know what you do.
