Hello Rushton:
Your question is very hard on my brain.
I had to go through my Reggae section to give you an answer.
If I were to buy only two reggae albums, I would buy
1)"Bob Marley and the Wailers Live!" 1975 Island Records ILPS 9376.
2)Third World "You've Got the Power", Columbia FC3774
"Legend" as described by Slipknot, is a fine compilation of Marley's work. I prefer "Live" because I am partial to recordings of live performances, and I prefer the slightert faster tempo at which the songs are played on "Live".
Third World gets a nod, because while the entire album is good, "Try Jah Love" (Side One Track One-written by Stevie Wonder) is an ultimate test of one's groovenosity/life force quotient. If you don't dance or at least get your head bobbing when this track is played at a good volume, you are likely in a coma.
However, don't obsess over the choices. Your approach, IMO is unimpeachable-buy two, play them to death and get them inside you. Then buy just one more/Repeat process.
I have to go blast Third World now.
Happy Listening.
Michael