My very rough rule-of-thumb is 5hp per foot of width, i.e. a 5 foot snowblower requires 25 hp. My tractor is 24hp and it handles my 50" snow blower with ease - except when I get into hard packed drifts three feet or more in depth.
The type of snow and what you want to do with it are important factors. Wet snow is much harder to throw than dry snow. Two-stage snow blowers will usually throw snow further than single-stage snow blowers. If all you want is to dribble the snow a few feet off the edge of the driveway, a single-stage snow blower will do. If you really want to throw snow, you should have a discharge velocity of about 5000 feet/min. My snow blower has a 20" rotor and I run my tractor engine at 2000 rpm which gives me a PTO speed of 1000 rpm - this results in a discharge speed of just over 5000 feet/min. I can throw really dry snow about 100 feet but some wet snows will barely go 30 feet.
A critical factor in determining how well a snow blower works is the gap between the rotor and the housing - it should be no more than 0.020" anywhere from BDC to the point of discharge. This quality of fit is typical in better quality snow blowers but may be lacking in lower quality units. The tips of the rotors do wear so I rebuild the tips on the rotor every other year to make sure I have an appropriate gap( i.e. build it back to 0.020"). A quick way to check the gap between the rotor and the housing is to put a dime at the bottom of the housing. If the rotor blades pick up the dime and carry it to the point of discharge then there is a good fit. If the dime falls down between the rotor tip and the housing, the fit isn't very good.
Another factor that will greatly affect the cost is whether you want a front mount of 3ph mount unit. 3ph mounts are much cheaper but you are always trying to look behind you. There was a direction of rotation issue for the PTO on some of the older Kubotas - check to make sure the snow blower is suited for the direction of roation of you PTO.
Finaly, snow blowers throw snow and not all of it goes where you want it to. A considerable amount blows back toward the operator - hence a cab, especially a heated cab, is a big bonus. Without a cab you will look and feel like an iceman in no time. I know because I've been there. The attachment shows my setup ready for action. Hope this helps. /forums/images/graemlins/smile.gif