Could I put a Seagat Barracuda 7200rpm 1.5TB internal HD in an external case?

Yes, I don't see any issue with getting a caddy for an internal drive. Seagate are good drive manufacturers, as are Maxtor and Western Digital. iOmega and Toshiba are relatively new to the HD market, and as such I couldn't really comment on their performance or reliability without testing them in 'real-world environments'.