Media Monster -Seriously...D TRE - What is a good heavy bag for beginners?What's your height/weight?
I'm 5'11, 180/195-fat, been doing Muay Thai for some time and anything under 150lbs doesn't give me enough resistance or difficulty for a good workout.
Then you have to ask yourself canvas or leather. Leather is easier on the knees and elbows if you plan on drilling those.
Will it be standing or hanging?
...and if you're a beginner I would recommend checking out a kickboxing/muaythai/boxing gym and getting some work in there before committing to a purchase.