If at all possible run channels through which new wires could be pulled. That way, as inevitably it will as new cables are discovered/designed to be far better, you can update them by having them pulled through the channels.
Is there any way to run the cabling under the floor vs. through the wall and behind the fireplace? That might save a lot of work/problems. It might also prove quite a bit less and provide a bit better sound if the cables are much shorter.
In answer to your question, no, concentrating your money on one or two cables will not ensure that you will end up with the best performance. It depends on the quality of the cables, period. Some extremely high priced cables are so-so in terms of performance while others which are considerably less are stellar.
It is entirely possible that one could buy an entire set of cables for the price of one or two very pricey pairs which would outperform the set with a mix of very pricey and less expensive cables. In short there is no guarantee that you will achieve the best results possible.
However, if you must choose and will not be able to later redo the project (i.e. if you can't run channels or go under the floor) then moving up the line on those cables you cannot replace at your liesure is a reasonable move.
Finally, if you have a rig set up now you can compare cables if your dealer will let you. Borrow a set of the AQ and of some others and do your own listening comparison. You also can obtain on loan cables from the Cable Company.
If this is a big deal/big ticket expense, which it seems to be, I would strongly recommend you slow down, do not let the dealer set the pace though you are wanting to support his business, and do your own due diligence on cables. You will not regret it. OTOH, if you are pushing a deadline then you simply have to make your choice and live with it.
Remember, this is a good problem, one of the sort that does not lead to suffering - unless you overspend badly! ;)
