If it is rusty it will be obvious.. the front crossmember rust is the common theme I am afraid.
Mine at the moment is very clean but the crossmember was shot..and had to be replaced
Anyway just do it
Inner and outer sills, front crossmember, either sides of the back arches, front left/right floorpan and rear left/right floor pan. That's everyplace I've ever had to weld
It should tow fine in neutral although when we towed back my old auto it may well have been causing damage I wouldn't notice as the box was already knackered!