Amount of filtering depends on the amount of fish, not just the size of the tank. Of course bigger tanks usually have more or bigger fish, but if you set up your 125gal tank, and put 50gal of internal filters in it, and stock it like it was a 50gal, it will work.
The thing with big tanks is that enough internal filtering tends to take up a lot of space, so people move the filtering to canisters or sumps to get it out of the way. But very few things are impossible. Some big power head sponge filters and internal filters will work, you may just end up with 120gal of tank and 5gal of filters, but so what?
Take the internal filter that fits in a 25gal tank, make it 5 times bigger, and it's a big ugly bucket, so they put some pipes on it and call it a cannister so you can hide it under the tank stand.
Plants make a big difference too, they can be huge nitrogen sinks and drastically reduce the need for power filtering.
So set up your tank, go with lots of plants, and understock. You can still have a nice looking easy care tank.
Ian