Several gotcha situations here. Unless you can get dispensation from the inspector, the drum trap and the 1.5" line really needs to go along with any of the galvanized you have access to while dealing with things. Drum traps are no longer legal, are impossible to get a snake through, and are often full of crud and ready to leak or clog. Since you're changing the function from a tub to a dedicated shower, the inspector may require you to bring anything you touch up to current code.
While I like surface membranes, I have no experience wiht the Latticrete stuff. It might work, it might not.
Being a multifamiliy dewelling, both code and the condo rules likely require any of the plumbing work be done with a permit and inspections. While you can probably dismantle things on your own to save money, there's a limit on what you can do when building it back up. Having common walls and floors, any glitch you may have or create impacts not only you but potentially many other owners, thus the requirement for limitations on what can be done by the homeowner. It'd be a different thing if it was a single family standalone home.
Now, getting access to change the drum trap and hooking things up to a viable 2" or larger connection may be tricky. Also, some larger buildings don't allow pvc and require all of the drains to be cast iron. This is generally not something a DIY'er is up to. Don't dig yourself into a big hole...both the inspector and the condo association could give you big grief. All it takes is one owner calling it in or a leak that they trace back to your errant work (not saying you couldn't do good work).