As noted, you should listen to Klipsch setup and also listed to other setups in that price range available for audition in your market.
For a variety of reasons I ended up with Klipsch setup in my expat HT, and while that setup is enjoyable, if I knew I would keep it for a longer period of time I would have made different decisions. If you go with 8000 towers and set them up appropriately they will pump tight and impactfull bass like there is no tomorrow, and that is the most fun feature of these speakers. They will be a bit harsher in the mid-high range which can be EQd to some extent but not fully. 504C could be the centre to match, but again, it will be harsher in the mid-high end range. I had it at one point and recall had to roll it off starting at 4khz as sibilants were pretty harsh otherwise.
Don't really see a reason to go with 6800H over 3800H if you are not planning expansion to 13 channels, or have front wides on your list as a must-have. For most purposes you would not need an external amp, but if you do want it, probably worth looking in some solid performing D class multi-channel amps. They don't have to cost arm and a leg and their power requirements would depend on the speakers, room and desired SPL. Most people don't really need more than 120-180Wpc into 8ohm.
Good luck with your project and hope you will enjoy it greatly.
