Probably the most popular tiny house sink is the ikea lillangen https://www.ikea.com/us/en/p/lillangen-sink-white-70207144/
I like this one better, it is a little bit bigger. Close to regular size sink, just removed the backsplash and space for the faucet behind it. https://www.ikea.com/us/en/p/hagaviken-sink-white-20324503/
Definitely block the walls for handrails while the studs are open. The other option is to use 3/4 plywood as wainscot to get solid anchor points, but you lose room in an already tight space.
I usually grab a roll of tape, an actual toilet, and a box the size I think I want the sink cabinet and do layouts on the shop floor.