Atmas can be worthwhile simply because it also provides a hefty chunk of armor. But getting autoattack items that don't contribute to Cho's survivability isn't worthwhile. His natural ASPD is just far too low.
The best items for a "dps" Cho'gath in my opnion are Phantom Dancer (dodge, movement), atmas (armor) and frozen mallet (health). Gives him about 300 DPS before armor with spikes. But you should only get those items if you are A) winning easily and/or B) the only player on your team that is successfully killing opponents. And even then you had better get a warmogs, tank boots and some other tank item (preferably angel) first.
Cho'gath should not have early game mana problems unless you fail at Cho'gath. In the mid-endgame you just need to keep a reserve of 350 or so for any given team fight, not hard since you can easily farm with autoattacks.
