Yes you will need to upgrade your slaves with both wow and tbc to reach 70 then WOTLK to hit 80!

If wow+ Tbc costs $130 it will cost you a total of $520 but you will recive 1 month free of play on your slaves and 1 month free of play on your "main" account as soon as you paid your first month on your slaves.

You can run wow from the same folder but its not recomended even thought i cant remember the reason!