Yep, those mounts have worm periods of 6-8 minutes. You might get one or two 100 sec exposures in a flat part of the worm, but there will likely be a quarter of the worm cycle that heads off with significant periodic error, than the next 1/4 taken up heading back the other way.
Both my EQ mounts have periods where a 2-3 min exposure unguided is possible, but no way you get away with it for the rest of the cycle!
Marc's right - best to go to "dark side" of autoguiding
You may have to purchase the odd bits and pieces here and there as you get further into astrophotography though
