I currently have 2No 140ah Varta batteries that are at least 5 years old, when should I look to replace them and would it be ok to increase the size to a pair of 180ah?
When the capacity is down to less than you need.
The lifetime of a lead acid battery is depending on the number of cycles, depth of cycles, temperatures, charging methods, deep discharges without immediate recharge and so on.
5 years is a mid value but no average value.
If you have two 140Ah-batteries wait until one will reach its end of life.
We are just replacing our batteries after 12 years because one of the two 140h-batteries died. A very good life time! In this case the 12V-system can continue to work with one 140Ah-battery. Disconnect the bad one (it's warm) and replace always both of them.
180Ah batteries would work BUT their dimensions will not fit.
Best regards
Frank