I would suggest going used if you're starting out, personally I'd buy a walk behind and save $3,000-$4,000 over a ztr or stander. A used walk behind can be easily found for $2,500-$3,000 with low hours (around 100).
About the size, that's up to you, since you already have a few push mowers, you may just want to get a 48",52" or up, and then use the push mowers for gates and back yards.
Also, use the search feature on this site and you can pull up TONS of old info.
I hope I could help, God Bless you and your family!
If then God so clothe the grass, wich is today in the field, and tomorrow is cast into the oven; how much more [will he clothe] you, O ye of little faith? (Luke 12:28)
For if we live, we live to the Lord, and if we die, we die to the Lord. So then, whether we live or whether we die, we are the Lord's. (Romans 14:8)