Depending on how often you mow will have the biggest bearing on what sized mower you should buy. Finish mowers are meant to be used on a regular basis to maintain the grass, but if you let the grass get too tall, they require allot more power and don't do as good of a job. When sizing rotary cutters, the rule of thumb is one foot of mower for every 5hp at the PTO. With a finish mower, you can add a foot to that without any problems, which means a five foot mower should be fine. Anything bigger might work, but only if you keep it cut on a regular basis and take your time cutting.
I wouldn't worry too much about blade speed and blade overlap. Both have been figured out by the engineers when the put the pully's on the mowers, and to the best of my knowledge, all will be pretty similar. Keeping your RPM's up to the recomended speed is all that really matters. If you get into grass that is too tall, you will either have to speed up the engine RPM's or slow down the speed of your tires.
Cutting speed will either be determined by how tall the grass is compared to your HP and the size of your mower. Another factor is how smooth is the field? My Zero Turn mower is very fast at cutting a lawn, but I can only go about half speed in allot of areas because it's just too rough and beats me up.
Both my finish mower and rotary cutter are Landpride. They are good quality impliments with a long history that's hard to beat. Of the other brands, you'll have to see what's available in your area and then do a specific search to see if there are any problems. There are some that people have complained about, but if that brand isn't sold locally, it doesn't matter.
Be sure to buy something that you can get parts for. Belts, wheels, and blades all wear out. Other things break, but mostly from a mistake on your part. I busted off a spindle on my mower and had to replace it. Having a dealer close buy really helped in figuring what the right parts were and just getting what I needed instead of buying a total kit for twice as much money, when I only needed a few parts.
My mower is a side discharge. I've never seen a rear discharge in actual operation, so I can't comment on them. I like mine, but it does spray allot of material out to the side. I then mow that material again when I make that pass and eventually it all disapears. I've read stories of people claiming that side discharge mowers leave windrows. Mine doesn't, it spreads the grass out evenenly. The longer the grass is when I cut, the thicker the cuttings are. It's just a simple equation of where does it go when it's cut, and how much is actually cut?
Be sure to get four wheels. The beauty of the finish mower is that it rolls over the terain independent of the tractor. Those four wheels make sure you have the smoothest, most even cut that you can get. Mine came with tires that take air. I had allot of flats, so I changed them to solid wheels. If you have a choice, be sure to get solid wheels. Flats are always a terrible way to spend the day, and repeat flats are just plain annoying. My tires were also worn, cracked and in need of replacing. New tires might have cut down on the flats that I was getting, but since I had to buy new tires anyway, I went for the solid ones.
Good luck,
Eddie