Is it worth the effort for a weekend logging project? How big is your tractor and how are you going to hold the tractor in place against the pull of the winch? You obviously need the engine running for the PTO so you can't leave it in gear. Most tractors I have dealt with over the years, didn't have that good a brakes to begin with. If it is 4X4, you probably don't have brakes on the front wheels so there goes a good precentage of your available traction force. Does your winch design include bars or framework that dig in to keep from dragging the tractor?
In my experience, the only times where a winch might be usefull is when I couldn't drive the tractor a reasonable distance to pull the log or other objects without having to re-rig the tow. In these instances I have always been able to rig a block to another tree to allow me to pull from a different angle than the log is being drug at. Also in my experience, winches are better at moveing the thing they are mounted on than the thing the cable is attached to unless there is a very definite weight/traction advantage.
With the cost of the winch you will probably be over $1K once you add in the price of the structure materials and tackle. There is of course the time to fabricate it and get the bugs worked out also. If you have room to move the tractor around, you might be money way ahead to just invest in some good tackle and use the tractor drawbar pulling ability to pull the logs, especially if this is only a one time affair. All I typically need is a long enough cable with good swaged eyes to reach from where I can drive to where the log is. I use that cable to drag the log in to where I can drive up on it for a short tow. A snatch block and chain for rigging is handy for where you have to rig angled pulls and some long chains for rigging the short pulls once you get the downed tree close enough to back up to. This is just based on my personal experience and I don't know your exact situation. I figure that if you have to drag a cable out to the log anyway, you could probably just as easilly pull the log with the tractor as you could with a winch. If you do go with the winch, just be sure that the cable pull point is below the rear axle(preferably down near the drawbar height) to eliminate the possibility of pulling the tractor over backwards.
Good Luck.