Tractors are inherently unstable operating on sloped ground. Tractor rear wheel/tire spread, sometimes adjustable, is a critical factor increasing compact tractor stability working sloped or uneven ground. A 6" to 10" increase in rear axle width substantially decreases tractor rollover potential. Tractor width is an approximation of rear axle width.
If your 15 acres is hilly consider a tractor of 3,500 - 4,000 pounds bare weight. Weight and width will provide a much wider safety margin on hilly land.
When considering a tractor purchase bare tractor weight first, tractor horsepower second, rear axle width third (stability), rear wheel/tire ballast fourth (stability and traction).