I havent seen a 4.5 US-delivery that didn't have A/C so since the m116 and m117 are so similar I'd imagine the compressor & mount are direct bolt on. The A/C shouldn't be too hard to install, it's finding all the bits in good condition - including the lower dash and kick-panel that goes below it that I don't think is on non-AC vehicles.
In fact there's a car on ebay being parted, a bit delusional about what the non-running engine is worth (as I've seen them sold for under $50 running)... but you may want to contact him about the whole A/C system:
http://cgi.ebay.com/ebaymotors/Mercedes-Benz-200-Series-280SE-4-5-Brown-1972-280SE-4-5-Mercedes-Benz-parting-it-out_W0QQcmdZViewItemQQhashZitem19b908ce2cQQitemZ110478544428QQptZUSQ5fCarsQ5fTrucks#v4-35