A lot depends on what your style is for the mtb pictures, but I'd say that the 18-55 you have is a suitable sort of focal length for bike racing, I've done a bit of cyclocross photography and found it to be a suitable zoom range for this kind of shot. The standard kit lens isn't the fastest, so something with a bigger aperture may be the answer. A lot depends on the lighting conditions you're shooting in. Wouldn't recommend the Sigma 10-20 for mtb work, it does make things seem very, very distant in the frame. By the time you have a frame filling shot of a mtb at 10mm, he's hit you!
If you get a decent flash, then it could be used on camera for a bit of fill-in with the mtb shots, and also used off camera with a brolly for portrait work. It all depends how seriously you want to take it. I use a Metz 58AF-1 wirelessly off camera and can highly recommend it for small scale lighting with a brolly that would work well for head/shoulders. If you want full length portraits, I think you'll have to get a studio kit.
Before you buy anything, I really strongly suggest you think about where you want to go with your photography. It does seem you haven't really decided what sort of work you want to do.
Duncan
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