I have two 125g tanks up at the moment.
Both have been up over a year, closer to 2 years for my cichlid tank and I have not had any bowing or other issues with either one of them. The 125g reef has had over 300lbs of rock and 300-400lbs of sand in it with no problems. The reef tank also has a center support brace sagging and a large gouge in the corner of the glass in the back and still no issues.
Knock on wood! lol
Just make sure that if you buy a 125g used that the seals all look good and it';s in good condition. If the tank is taken care of then you should be fine.