I am sure that the ATV/pull mower setup you are considering will do the mowing job that you need. If you plan to mow regularly, then a pull behind finish mower may be what you want; otherwise, a pull behind rotary mower would fit the bill. The only problem that I see is mowing across hills where the mower may have a tendancy to slide downhill...hopefully not dragging the ATV with it.
A tractor with finish mower and/or rotary mower will also do the trick. A large lawn & garden tractor will also suffice if you plan to mow frequently. You can also use the lawn & garden tractor rather than the ATV to pull the offset pull behind mower for double (or more) cutting width.
Other than money, the key issue that'll drive you towards one of the options over the others is the use of the equipment other than mowing. Depending upon the deal that you come across, the tractor is the most expensive but also the most versatile when it comes to keeping up the acerage. Most people find many uses for tractors that they had never thought of before purchasing their first one. However, if you have a desire for ATV fun, then that desire may override the more "practical" aspects of the tractor. Certainly, the cheapest option is the lawn & garden tractor which should last 5 years or more while you gain experience in this area.
This is a good board which should overload you with information.
Good luck,
Kelvin