A tilt shift is not good advice for the OP.
The 6D and 17-40, a Manfrotto 055 with a 410 geared head plus three flashes, I'd suggest YN-560 III's and a 603 II trigger, couple of stands and a Justin clamp. Oh, and this eBook http://photographyforrealestate.net/lighting/
For the vast majority of real estate shooting size and absolute IQ is not a factor, MLS limits are often very small and 1024 px images are normal, any advantage a tilt shift can give could be done in post to these sizes and larger. The only time a tilt shift is a good buy is if the clients are demanding higher quality for posters and architectural images, magazines, things like that.
At this point in time the majority of my income is from real estate photography, specifically for developers, they demand high IQ as they output in print and posters etc as well as websites. I use the 17 TS-E, mostly for interiors, but don't require the movements that much, exteriors normally need a longer focal length.
As for shift use, well as Don says, in real estate it is mainly used for keeping walls straight when shooting taller lines, it doesn't actually change the plane of focus any, that is always parallel to the plane of the sensor until you use tilt, but tilt has very limited use in real estate work, much more in architectural imaging. I use shift to get less ceiling and more floor space in the image whilst keeping the optimal viewing height, but you can fake that with a wider lens and crop.