A good zero turn? That's a good question but with as many people out there, you will get as many opinions. If you think about it for a minute or more you just may come to the conclusion I did. Look at the top four or five mower manufacturers and see what they have in common. Kohler, Briggs and Kawasaki engines offered on all. Parker or Hydro Gear pumps & wheel motors. What's left? The frame, deck, electrical system, wheel & tires and creature comforts. Perhaps a few items I didn't mention but minor stuff. Since the engine and transmission are all about the same I looked at the seat, warranty, replacement part prices (blades & belts) and actual construction. Some of the Snapper decks have depressions in them around the spindles and that is good for nothing but holding some water to prematurely rust the deck. I absolutely hate vinyl seats, Grasshopper uses cordura and about 4" of padding and an open area where the seat back meets the seat bottom for better circulation. I could list many features & benefits but you will have to go to the dealers and do your own research. I am overwhelmed by the quality of my Grasshopper and wouldn't even consider another brand. I have worked on Exmark, Gravely, Scag, Snapper, Toro, Ferris, MTD products and many others I can't remember right now and that has allowed me to take an up close and personal look at the differences. If Grasshopper didn't exist I would consider Exmark.
I do contract work for the largest lawn care company in the area and the owner was using Exmark mowers for years until he switched to Grasshopper mowers. He commented that he is having fewer problems and break downs with Grasshopper vs. Exmark. Don't get me wrong here, there have been some problems/issues/break downs/etc. with Grasshoppers but fewer comparatively. Doing some simple math, the average homeowner will put 40 - 50 hours a year on a mower. His crews put over 100 hours a month, 9 months a year. In other words, his four year old mower in reality is a 40 year old mower, hours wise. After seeing what his mowers go through and how well they held up, I purchased a Grasshopper 223 with a 52" deck. It's not the fastest mower out there (9.5 m.p.h.) but it sure does get after it. Personally I wouldn't want to go any faster because my multiple-acre property is a little rough and it gets a little bouncy sometimes. If you want fast get a Dixie Chopper or a Scag (up to 16 m.p.h. I'm told) but higher speeds are completely unusable around here except for going from point A to point B across the parking lot. Another major factor around here is Bahaia grass and Grasshopper decks are superior when it comes to cutting Bahaia on the first pass.
A huge factor to consider is dealer support and ranks near or at the top. If you can't get parts or service the best mower in the world will eventually become useless. And that brings it all back to the fact that you must go out and talk to all the dealers in your area and find one with a product you are satisfied with and the dealer supports. I know I didn't answer your question directly but if I did someone else would come along and tell you & me why I'm wrong. Again, lots of opinions out there but experience screams above all. Stay away from zero turn mowers priced $5,000.00 and under. The reason they're cheaper is because they're cheaper. JMHO.
Now, about your tractor style mower with no deck. I was given a John Deere L120 with a broken front axle. I fixed it for $50.00 and use it to pull a leaf sweeper and garden cart. I also loan it out when I work on mowers for other people. I have extra trimmers and blowers for the same reason. I like to provide a little extra service and when I take in a trimmer I hand them a loaner to use. The look on their face is priceless and the word-of-mouth advertising is even more valuable. Okay, I know this was long-winded but for some reason I just felt like saying a little more tonight. You can do whatever you want with your old mower. Some people advertise on craigslist to sell parts, some people take it to the scrap yard for cash.
Good luck and let me know what you decide.