Some things have already been mentioned above, but I will go over the items that I check out when I'm looking to buy one.
*Check all forward gears and the reverse gear to make sure it goes in and out of each gear, and stays in gear while under load. You want a little resistance for each gear, and if it goes into each gear with little to no resistance, then you'll have to look into splitting the rear and installing new detent balls. In most cases, if the shifter goes into each gear easily, then nine times out of ten, the shifter will jump out of those gears just as easily under any type of load.
*Check the variable speed control. Start the tractor up if possible, and put the transmission into 1st gear. Pull the variator handle all the way back to the rear position and let off the clutch/brake pedal. The tractor should not move. Move the variator handle front one notch at a time to see how the tractor speed reacts. Each notch should give you a little faster speed.
*You don't say what year the tractor is, and you don't specify as to it being manual lift or hydraulic lift. If hydraulic lift, be sure to check it out thoroughly to make sure it moves and to see if it lifts one of the attachments. Rebuild kits are still available through Deere, and you have to buy (2) separate kits in order to have a complete rebuild. One of these kits are quite expensive. Check the hydraulic hose lines for damage, splits, bent fittings, or dry rot. The hoses can be pricey when it comes to replacement, and are not available through Deere anymore.
*Try to see if the starter/generator is good and strong while starting the tractor. If it sounds weak or like it's barely turning, then you'll have to think about replacing it or rebuilding it. Not expensive, but yet another expense. Does the generator light on the dash (again not knowing the year, as 1963 and 1964 models didn't have one) stay on while the engine is running at medium throttle. Could be the voltage regulator will need replaced. Again, another expense.
*Are the tires decent, recently replaced, or dry rotted to the point that they will need replaced? Do the rear rims show rust stains around the valve stems, due to the tires being filled with calcium?
*Are all guards in place like the PTO cover on the right hand side of the tractor, or the starter/generator shield on the left hand side of the tractor? These items aren't available through Deere anymore, and will cost some big bucks when you find them listed on ebay.
*You mentioned a tiller attachment in your opening post. Since this is a 110, look at the mule drive hanging below the frame just to the right hand side of the tractor. There should be a pulley on the inner most part of the mule drive, and two pulleys on the outer most part of the mule drive. If only one pulley on the outer most part of the mule drive, then you won't be able to use the tiller since you don't have the second pulley. Look at the back of the tractor. Is there a step type bracket bolted to the bottom of the rear hitch? Is there a finger like piece coming out through the hitch plate itself? If these things are not present, then the tiller was never mounted to this tractor. If these things are present, then look under the tractor to see if there is a large, adjustable spring that expands when you push the lifting arm front. This would be a helper spring, and will make things a whole lot easier when trying to lift the tiller. The helper spring would be an added bonus!
*If the mower deck is installed, start the tractor and activate the pto to hear how the mower deck bearing sound. Quiet as possible is what you want to hear, but if noisy, then you'll eventually have to replace the bearings. Does the mower deck have any holes rotted through it, especially the front left facing as this is where the grass always gathers and rots. The right hand mower deck wheel will more then likely be re-welded on, and this is normal. Bad design from John Deere, so don't let this keep you from buying the tractor.
*Does the tiller turn freely and what kind of shape is the drive belt in? Is there an adjustable turn buckle present that connects to the finger type piece sticking out the back of the tractor hitch? Is the leveler blade/deflection shield present. Another words the flap that hangs off the back of the tiller. Is the main chain drive guard present, or is the drive chain exposed and all rusted up?
Other than the motor, these are major things that I look at or take into consideration when buying a round fender. I wish you luck on the purchase, and like said before, post pictures of it when you bring it home.