Unfortunately I think the answer is: It depends. I think you could definitely get away with what you have. Make sure the baffles are low enough that you have enough room in the sump to hold all the water that will drain down from the 180 when the power fails.
However, typically I find that no matter how big my sump is I always wish it was a little bigger. If you're planning to use an internal skimmer or any reactors those will take up sump space. With a larger sump you might be able to have a fuge section.
There really is no standard rule for a proper size sump. As long as it is big enough to hold the water that drains down in a power failure and still holds enough water to keep the return pump wet when the power is on and as long as it holds all the equipment you want, then it's big enough.
Typically replacing the sump is a reasonably easy job, unless your stand is such that the sump will be locked in once the tank is in place. Assuming you can remove and replace your sump after the tank is set up, then I would suggest you start with what you have and see how it works for you. You can always spend the money and upgrade later if you decide it's too small.
On the other hand, if you would need to drain and move the tank in order to replace the sump then I would suggest going with the biggest sump you can get in there now. You won't regret having extra space, but you very well might regret not having enough space.
