Hey, welcome to TPF. If you get the 5D-II, then the 18-55 lens will be of no use on that camera. The difference between FF andf APS-C is a BIG one at close distances: there is less depth of field on FF than there is on APS-C for the "same picture". That means where you can shoot at f/2 on the XTi, the exact image on FF will likely have an out of focus chin or mouth or eyes or forehead, depending on how the shot is framed.
I'd buy a BIG reflector and a grip arm to help aim and stabilze it; a rectangular reflector, or a square one, or better yet, two, exactly identical reflectors so you can wire or tie them together, and make a free-standing reflector that will NOT absolutely require a stand and a grip arm and clamp. Round reflectors do not work well for this. With two identical rectangular or square reflectors, you'll be able to make a sandwich board ( capital letter A shaped) or a V-flat (hardcover book standing up on its bottom, opened in the middle) so you can bounce a big swath of light into the shooting area, while working without an assistant. Or fire a flash into the V-flat!
You can do a lot with a 50mm lens on Full-Frame or Crop-frame. On FF Canon, a used 24-105mm f/4-L IS USM lens would be good. I owned one, and this lens is widely available and is the only lens needed for 90% of family portraits. Yes, it's $600 or so, give or take, but it's very handy, and has the right maximum aperture to keep you from getting into jams with f/2.8 problems.
I think one thing the 5D-II would be great for is shooting High Speed Sync portraits, outdoors in bright sunlight, with wide-ish lens apertures, at fast shutter speeds, like 1/2000 to 1/6000 second, in that general range, with the lens at wide-ish apertures like f/2.8 or f/3.5. This is mostly done with longer lens lengths, and a small modifier on the flash, like the Rogue Flashbender mentioned above by JoeW, and an under-chin type of bounceboard reflector, and maybe another reflector off to the side and slightly behind the subject(s).
I think a person really, really, really needs to be fluent in how to use a speedlight or other flash. We've got good ISO cameras now, where ISO 800 or 1600 is VERY usable--especially in daylight or in open shaded areas...but if the eyes lack sparkle, often times the photos look bad. Buncing a speedlight into a large reflector, and using High Speed Synch can give that eye-sparkle very easily.
People are impressed with lighting gear: reflectors, lights, and stands. Every photographer has a camera. If you want word of mouth to grow, fast, use lighting equipment; it will make your pictures easier to shoot, and flat-out better. Even flopping a 42 x 78 inch reflector on the ground at the feet of two kids sitting on a picnic table will make the photos better!
This is not studio lighting: this is location shooting. Reflectors are mandatory. Flashes are almost-mandatory.