My parents have an old 8-hp Horse (model) Troy-bilt tiller. 

I bought a 5-hp Pony Troy-bilt tiller.

I use both of them.  For primary tillage in the spring, I would rather use my parents tiller; when it comes time to weeding between the rows, the 8-hp tiller is too big - I would rather used my 5-hp tiller, or weed in some other fashion.

The other thought that goes thru my mind - if you are going to buy a lawn tractor, would you want to get a tiller for your tractor?

With all that said, I agree with those that say rent for a year or so, until you know what your needs are going to be. That will give you an idea of how much you want to invest in gardening tools.