Congratulations on purchasing your first cub cadet!! You chose one of the best to be your cub, since the 123 was the very first one to have the hydrostatic transmission and be made of almost all metal parts!!! It should handle the 48 inch deck quite well for you (or smaller). Since you mentioned that you just put a new mule drive belt on and it is bogging down when you first start it up, you should check the tension of your belt. As a rough rule of thumb, inspect your mule drive pulleys on the undercarriage, located directly under the front pto of your motor. Does the belt form right angles on both pulleys?
You can reference our technical section of this website for further information to help you. Try looking in here for starters....
http://www.onlycubcadets.net/forum/showthread.php?t=5
(Thanks Roland for fixing the above link today!!!)
and then later here.....
http://www.onlycubcadets.net/forum/s...ead.php?t=4683
Happy Cubbin,
Cub Cadet 123