my tank is made of 5mm crystal glass and it hasn't caused any problems. the tank maker, n30 also gives a 5 year warranty and base on their works in marine tanks i do trust them. having said that my tank is only a 60x30x36cm tank. larger tanks would definitely need thicker glass of more than 5mm.
as for the original question on freshwater to salt, i did the same thing when i started.
took down my freshwater tank, added a sump, changed the lights, added a skimmer etc. if i could go back then and redo it i would have went for a larger sump straight away to house more equipments that will eventually be necessary. having a good skimmer at the start saves one the process and money of upgrading later on. extra space in the sump that may seem excessive at first is always better then not having any space later on. the increased total volume is also not a bad thing. as for the overflow, i would personally have more ease with a drilled overflow then a overflow box/pipe based on my experience with them failing