ok.
Spirit 6 can be mounted vertically as well. Since it is slim model, will fit well for your requirement.
Try to avoid mounting L/R close to the TV unit horizontal plane, if it is close there will be reflection from there.
The Center Spirit 8 you can keep close to the edge of the table so that there won't be reflection for that as well.
You need not consider XPA3 for Spirit 6, since the RMS wattage max is 125W. Possibly you can add XTZ Edge A2-300.
You can start with Marantz 6014 AVR, can add the Amp later (if you feel there is a need)