Get two....get Blue Train because of the reasons given. And also because Trane's improvisations are grounded in the blues and will appeal more to the average music lover than his later, freer atonal explorations. The second album to get, if you don't already have it, is Kind of Blue. It isn't a Coltrane album, it's a Miles Davis album. Coltrane was a side man on tenor in that version of the group, and together with Miles Davis, Cannonball Adderley, Bill Evans, Paul Chambers, and James Cobb, he created some of the greatest music in the canon of jazz.
After that I would suggest Cattin' With Coltrane & Quinichette, and by then you will be ready to dig into some of his recordings on Atlantic (Giant Steps, My Favorite Things, Coltrane Jazz)...and beyond.
--Jerome