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Based on the information you've provided, I would think a 30 hp would work well for your needs.
You mention that you will be mowing roughly six acres of pasture. For this, you would typically use a rotary cutter (a.k.a. bush hog, brush hog, shredder). This is a 3 pt. implement that is designed to do rough mowing of small diameter shrubs/trees, heavy weeds, etc. Once pasture areas have been mowed for awhile, the rotary cutter can actually do a reasonably nice job in maintaining the area and looking fairly groomed. However, the rotary cutter is not intended for "finish" mowing, such as on a typical lawn.
This is where the mid-mount mower (mmm) and/or the rear mount mower (rfm) would be used. The mmm deck fits under the tractor between the front tires and the rear tires and can be raised or lowered. It is the better choice IF there is a lot of close turning mowing that needs to be done. I believe that some mmm's can be found with rear discharge, which is an advantage because then you can up against an object in either direction rather than being forced to alway mow in one direction because the mower is a side discharge. I'm sorry that I can't tell you which manufacturer's offer rear discharge. The imprortant thing to remember with a mmm is to make sure to get a deck that is wider than the tractor's footprint so that you can get right up next to objects. Typically, mid mount decks are more expensive than the rear mount.
The 3 pt rear mount mowers are less expensive, easier to attach or remove, but do add length that you need to worry about when making turns. One of the common problems for a new operator is turning too quickly after running alongside an object and whacking it with the deck as you turn away.
Another advantage of the rear mount is that you can back it under a tree or fence to mow, while you couldn't do that with a mid mount. You also can get rear mounts in either side discharge or rear discharge styles.
I would think that if you expect to be using the tractor in a variety of ways, the rear mount mower would be more convenient just from the standpoint of ease of attaching and removing. That being said, the manufacturer's have been making improvements in the design of the mid mounts to make them easier to get on and off, although I would suspect that the rear mounts are still, overall, easier to handle.
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