hi and welcome to sts! i am on the tail end of meso 2 and love it.
how heavy you need to go really depends on your abilities and goals. i have had the troy lite set for around 10 years now, and i am just about to outgrow it. the capacity is around 80 lbs which is much heavier than i need for upper body at this point, but not heavy enough for me to effectively do lower body and get the most out of the system. for reference, i am not a super advanced exerciser.
thinking back, i would never ever have thought i'd need something more than what the troy lite can provide. in retrospect, i wish i'd have gotten something with a heavier capacity than the troy lite. i do love the 5' length; its convenient for working out in a small space without feeling like you're going to hit everything. i also really like the spring lock collars, super easy.
in terms of other equipment, i have up to 30# dumbbells and i am about to purchase 35# and 40# pairs in prep for meso 3. i also just bought the valor bd-9 squat stands since the cathe tower is no longer available, and benching 60# is just not very safe when you have to hoist it up and figure out how to get it back down to the floor from a lying position.
you will also need a step with a number of risers per side, or a weight bench to do lying exercises. you will also need a band, a stability ball, and tubing. and something to do pull ups from...cathe tower, squat stands with a barbell that has enough capacity to hold your weight, pull up bar for a door frame or the modified method of using the tubing. i believe cathe's tubing on the shop.cathe.com site comes with the piece needed to do that.
seems like a lot of stuff but this program is so awesome it is totally worth it. happy shopping and good luck with sts!