I am convinced that mice can walk through walls. :) They can and do get into everything.
If you buy used from a dealer, you may get some form of short term guarantee. However, you are buying a used unit, so other than everything working when you leave the dealership, there is little recourse on things that break a month latter. That is the chance you take for the money you saved.
As for construction techniques and/or poor workmanship, judging from the various shortcuts I have seen in my TT, the standard in the TT industry is a not what you see in home construction. There seems to be a lot of "good enough" thinking.
Good Luck