between the 2.5gal, 10gal, 55gal, and 120 gal planted tanks I have/had: the 120 has been the easiest to take care of. But also 2nd most expensive (I had a 60gal reef too). If you are patient, a big tank will fill out. You can start it low tech and have a wonderful tank quite easily. The most expensive part of a big tank (beside the tank/stand cost) is if you want lots of hardscape stuff. Rocks and sticks can be pricey.
But yea, your filter choice is probably no good if you will have that many fish. I run a Aquatop Forza FZ13 ("rated" 550gph) to a spray bar on my 120 and had to add a powerhead. Turnover is fine I guess but it's too deep so had dead spots. I'm in the process of switching over to a sump (tank was already drilled) so I can run a bigger return pump. And for a big tank, unless you have a RO unit in your house, do not even worry about pH. Test your tap, and pick plants/fish that will work there. You'd be surprised what can be done. You will pull your hair out chasing a pH level with that much water. We have crazy hard/high pH water here and even with CO2 on I can barely keep my water below 8.0, and my plants don't seem to care. Fish though, it's debatable. I have a slowly dwindling school of cardinal tetras, but they are all 2-3 years old at least (I inherited them with the tank over a year ago, bought used). I don't replace them though since I think it's just not right for them. I may gut my hardscape and ditch the stem plants to accommodate shellies once the cardinals are gone.