personally I do not think a sump is really required for that sort of tank set up.... Canister filters etc... would be sufficient
but if you want to make one good on you, and go for it.
126L is more than enough.
problems you will encounter though would be maintainting a decent syphon unless your holes are drilled at or near water level (being a turtle tank a lot lower than most tanks) and this may limit you in the future if you want you use this tank again.
seriously consider an external canister for this before you start drilling.
alse regarding evaporation, you should absolutely not have to worry about this as your regular partial water changes should replace this.
I'm assuming you are getting your sump info from saltwater keepers where evaporation is an issue due to salinity issues.
with saltwater tanks you will need to add extra freshwater to resolve this problem, but in freshwater PWC's take care of this.