No more overkill than the TZ. 18 hp is 18 hp. I jumped from a riding Gravely to a TC-24D (same frame size as the TC-18) 2 1/2 years ago and haven't used the Gravely since, not even around the house that sits on a tiny 58' x 170' lot. The TC handles like a dream, turns tighter than the Gravely, and really doesn't have that much bigger of a footprint. Take the loader off in minutes and you've got a great mowing machine. I have turf tires and a soft yard, and with the loader off, it doesn't leave as much of a track as the Gravely did.
I'd say go for the TC. There is not any difference in the size of PTO driven implements you can use, but the larger and heavier tractor will provide you better traction, ground clearance, and loader capacity. If storage of the bigger TC is a concern, the TZ will also be a great machine. Just don't let the size of your lot sway your decision - its the size of your jobs that matter.
Think about it, people drive Hummers and Chevy Suburbans for their daily city commute and wouldn't have it any other way /forums/images/graemlins/wink.gif
Good luck and let us know how you make out.