Depends on what you're doing of course, but for the way we use tractors weight is as much a limiting factor as horsepower. Without enough weight, the implements want to steer the tractor, rather than the other way around. For us this happens mostly with ground engagement. You can't draw straight beds if you can't steer. Weight in the bucket helps, but with small tractors the buckets are limited in capacity. I've been told on this site that 75 hp is enough for small scale farming, but I'll never own less than 120 hp again. But that's for berry farming, which involves using large tillers on a lot of acres, and also pulling a raised bed maker. For mowing you could get by with much less. Lots of riding lawnmowers fall in the 25 hp range. Problem is, though you can buy little implements for little tractors, most of them are not worth owning. This is especially true for box blades, etc., where weight is essential. Just my two cents based on my experience of course.