i by no means want to discourage you from going marine, if you already have the tank then so be it and you can work around it and make it into a marine tank...... however if you have not yet bought the tank then it's something to take into consideration
we've got a 120l tank and are currently working on setting up a sump for it, i would definately want one for a 190l tank. i'd want one for any marine tank really tbh, think it makes your life a lot easier and makes the tank easier and more stable to run. It would be a big enough factor to put me off buying one specifically for marine, just my opinion though.