Ah,depends if garage who MOTed it also sold it, in which case it could easily be duff.
But if a private sale lets assume MOT is "straight", so car is safe. On mine I have just found loose spring from door lock rattling in door, so there are rattles that don't matter.
I also had a Pug 406 which had loose drive shaft UJs, so that rattled on uneven roads, deep clunky noise. I think it was quite safe, maybe a common Pug fault. I sold it rattly for �300 or so!
Re water loss, if you overfill it will blow out the excess, see if it stops leaking from now on. I rate Radweld for leaks, but don't use it if there is no leak as it can easily partially block heater.