If you're using it for leaves I would highly suggest getting one that has its own engine that powers an impeller so as to add suction power. They're not cheap, my contraption cost right around 3 thousand but I have been so pleased with it I only wish I had bought it sooner.
Keep one thing in mind, leaves are hard on equipment so there will be added maintenance costs, the larger / heavier units (especially those that really hold a lot of debris as that adds weight) can be hard on the hydraulics of the mower. Don't forget grass is heavier than leaves, depending on the size of the "bag" you may have to empty it more frequently with grass, however smaller bags won't really save much time in the fall with leaves.

There are several different brands (and types) available...
The one I use is called a Cyclone Rake, it is a tow-behind bagger with its own 10hp engine.

My labor rate to suck up leaves with that bad boy is going to be around $90-$100 an hour this fall.

 

Just go buy a used GUS Walker Mower. You can get a good used for under $5k. Use it when you need to collect.
Then, after you figure out it mows so much nicer than anything you’ve ever ran, you get rid of your other mowers and switch to Walker.

 

While I have always been wanting to try one, we have zero dealers near us for many states Im told.

 

My only experience is with the 14 bushel dump from seat bagger on my jd 994. Huge improvement in productivity for fall cleanups, leaves a very nice manicured finish once done. Definitely quicker when emptying than messing with three bags. We'll either dump in the back woods, leave at curbside for town pickup or come back with debris loader and dump trailer. Three bags may come in handy if you want to manually dump them in dump trailer.

I believe Walker makes some highlift hydraulic dumps so you can dump direct into dump trailer if that's the route you go.