Seeing as I haven't had time to read the entire thread, I don't know if anyone else has suggested these yet - but in case they haven't, I'd suggest getting the Trojan Dub Box Sets (both vols 1 and 2) as they contain a wide and varfied selection and are also pretty cheap, considering they're both 3CD sets.
I'd also second Squid's recommendation of Lee Scratch Perry's "Arkology" box - it's an essential primer for anyome who's interested in the genre at all, really.