If it were me, I wouldn't start with a 75 and upgrade later. I'd upgrade now and use the 75 for a sump. You'll save yourself a lot of headache down the road and avoid the temptation of keeping fish in a tank too small for it longer then you should. I love predators, but a lot of the coolest predators are big and need a lot of space. Some people find smaller predator tanks rewarding, but 125 is the smallest to go with IMO.