Good plan to take it to the studs if you are planning a tile shower and radiant heat. I would even take up the floor around the toilet to inspect the drains, look at the shower drain, the sink drains etc and make sure it all looks good and no evidence of leaks. When you get to putting it all back together remember it will leak. Design every layer to deal with leaks, I used plastic on the thuds, then tar paper, then the Hardibacker which I sealed with Red Guard. One I built had a tub and the other was all concrete, the concrete one also had a butyl rubber type shower pan in there as well. Moving the placement of a shower or tub or sinks is relatively easy after you gut the bathroom, but moving a toilet is not as easy especially if you have cast iron lines.
Don’t forget to insulate around everything especially the shower, that will keep it warm and keep the water noise from traveling as far. Remember to upgrade your outlets to GFCI, install a good exhaust fan for the water vapors. Water is the enemy, it can cause a lot of damage and is patient enough to keep attacking things until it wins. That is why I say to plan everything out as if it will leak.
Good luck!