The point of use tank type ones are great. You can even take a quick shower with them, if you turn the thermostat fully up and have a low flow shower head. 2.5 gallon is perfect for a sink. If you want to take a shower, I'd get a 5 gallon one.
Don't have it poured when it's too cold or too hot. Pick a mild time of the year. I'd go a bit thicker on the slab then specs requires and carefully inspect the rebar before it's poured.
Use a magnetic stud finder to locate the trusses from the nails and screws. Blindly cutting or drilling holes is unprofessional in my opinion. The best thing about magnetic stud finders is they don't need batteries and will stay stuck on the ceiling or wall, while you are doing other things.