For real estate purposes I would not hold back on the Sigma lens - the difference in price between that and the Canon lens is significant!!! Very significant!!! I'm only considering getting the Canon one because I like to dream and I do love photography - it's my hobby along with my business. For my business the lens is sufficient - I just need to sharpen my images more than I'd like when I load them on the computer.
Regarding being a top agent and using a $100 camera - of course it can be done and I am sure your pictures are way better than a lot of the images I've seen online (OMG, I could write a book on awful MLS photos.) Pictures of the inside of refrigerators, rooms so dark you think that they're advertising homes for vampires, blurry shots, pictures with people and pets in them...it's just bad and it reflects poorly on our profession and helps make people question the necessity of a realtor...
What galls me is that agents who don't give a crap about the kind of pictures they put online not only get listings but sell them. It goes to show that when all is said and done, what sells a house is the price and the location, really. Excellent pictures help but bad pictures don't always hurt if the home is priced right and buyers decide to look at it.
Do I think you need to spend $1,000 on a camera and equipment? Not really. Do I think you should take excellent photos? Yes, because it helps buyers see the true picture of the home and it gives us the air of professionalism. I have had people contact me and asked me to list their homes because they love the pictures I posted on my blogs. It kills me when I see someone else get a listing I wanted and they butcher the house with awful photography....lol...it pains me!
Good photography is but one tool. If you're going to do it, I say you should do it right. My camera cost me about $500 or so when I bought it, the Sigma Lens was around $400....I already had my tripod and my 28-300mm zoom lens was a gift (but I still don't like it..lol). I could also do real estate just as well with the Lumix or Kodak point and shoots with ultra wide angle lenses....I just love my camera! I can't take the kind of hobby pictures with a point and shoot that I can with my DSLR....