Are you planning to use this system mostly for home theater use (movies), or mostly for multichannel music (SACD or DVD-A)?
Assuming the former, I think KEF 104.2's would be gross overkill as side/rear surround speakers, and they may actually be poor choices for at least three reasons: 1) They need at least 6 feet, preferably 8 feet or more, to sound their best -- do you have that much distance between all listening positions and the side/rear speaker locations? 2) The side/rear speakers are frequently mounted a few feet above ear level -- this would be difficult with the large, heavy 104.2's. 3) The 104.2's like to be powered by plenty of clean, high quality watts, which the Marantz receiver may be unable to deliver, especially if it is the sole source of power to all 7 speakers.
I have been very happy with my 104.2's as front main speakers for 21 years. I have used a 100C for the center channel for 15 years. Although others will say the 200C is a better match, I have been perfectly happy (except for some age-related deterioration at age 12 that I repaired myself) with the 100C, at less than half the cost of the 200C.
I use a pair of KEF Q2ds, mounted on the walls a few feet above ear level, for the side surrounds. They sound fine, but I was disappointed by their fake woodgrain finish, which the IQ60c uses as well. If your center channel speaker will be in a visible location, then you want it to be as attractive as the 104.2's with their real wood veneer.
I use a pair of Paradigm Titans, mounted on stands slightly above ear level, for the rear surrounds. They have been trouble-free for 15 years.
I believe that the "conventional wisdom" of using identical speakers in all surround locations is overrated and unnecessary for home theater applications, now that the receiver/processor automatically calibrates the volume of all speakers, and frequently does room correction on them as well. I suggest that you get some small speakers for the side/rear surrounds, either vintage KEF 102.2 if you want to stay with KEF and can find them, or any decent bookshelf or wall-mountable speakers from a reputable manufacturer for no more than a few hundred dollars per pair. For the front center channel, I suspect that most dedicated center channel speakers, costing no more than a few hundred dollars, from any reputable manufacturer, will be a good match with the 104.2's.
I personally like dipole speakers for the side surrounds and monopoles in the rear, but others prefer monopoles in all locations.
P.S. -- Hurry up and build those subs! Enjoy.