I'm using M&K S150's across my front. They are awesome speakers. I really can't begin to tell you how much I like them. However, unless you plan on adding S150's to your L/R, it may not sound right with what you use or eventually use. Timbre matching the fronts can be very important. Also, it is a bit intrusive at times. The depth of that speaker is around 1', actually its about a cubed foot in size. Also it is somewhat heavy, ~23lbs IIRC. You may want to make sure you have either a way or somewhere you can mount it before you get one. Try making a dummy box with some cardboard to make sure you can fit it into your system. When I first got mine, I had no where to put it. Now it sticks out in an intrusive manner, but sounds so good I don't mind.
$650 is a reasonable price for that speaker from a retailer. They usually sell between $500-700 used on ebay (retail was $1100

) As far as is it being 10 years old, it's hard to say. M&K started making that model ~'96, that doesn't mean the one they have is 10 years old, it could be newer. Those speakers still come up for sale on a regular basis. That deal is not good enough for you to feel you have to buy it now. You have time to decide and if you still want M&K's they will be found.
Again, buying a center that doesn't timbre match your fronts will likely irritate you by not sounding right. Buy the center that will match your planned fronts.
Jack