Replacing through the wall air-conditioners

[Robert Nason]

I'm looking to replace 12 older air-conditioning units that are through the wall models. My HVAC guy is telling me that I cannot use conventional wall unit air conditioners and the one that he is proposing will cost me around $1000 each. With everything being equal in terms of BTUs and dimensions I can buy them for half of that price at one of the big box stores and install them myself IF it's as simple as sliding them into the existing sleeve. Any information would be greatly appreciated.