I just sold a 180 that I owned since 87/88 and it served me well. IIRC the 170 is just lower HP but basically the same machine.
Check the deck very close. On the back portion of the deck on the skirt, directly in front of the right rear wheel, on the inner side where the blades have a tendancy to concentrate the dirt and clippings at, especially. The entire deck can look fine, but that area gets eroded away and can be paper thin but gives no indication of it being that way. Just tap on it with a ball pien hammer using the rounded portion of the hammer, and if it dents really easy its getting pretty thin.
Spindles are another issue I had problems with, especially when I followed JD tech advice on installing new bearings. Shake yur blades up and down and side to side, and if there is any amount of slop the bearings are shot, and should be replacd, before they seize up and wallow out the spindles cast aluaminum housing. Another giveaway to bad bearings is the deck makes a roaring or rumbling noise. These housings are usually drilled and tapped and have a Zerk fitting. However the bearings used are double sealed, so you actually get very little grease if any at all in the bearings themselves when you grease them. Pull the spindles apart, and remove the bearings. Even if you use new bearings, remove one seal from each new bearing before you install them.
John Deere says to install them with both seals, just as they are . I have to dissagree. Install these new bearings with the side the seal has been removed from so they face each other and are located in the housing itself. NOw when you re-grease them new grease can redily flow in and push old grease and any grit out of the bearing and past the seal to the outside of the housing assembly. With inner seals intact its not able to do this. You may get "some" grease inside but not enough to flush out any crud etc.
The overcenter lever for the mopwer deck to PTO has a tendency to wear a notch in the spring and lever as well as the attachpoint on the deck, and the bushing in the deck idler / tension pulley bracket also is a high wear item. I replaced the plastic JD bushing with a oil lite sintered bronze, and it lasted lots longer (Years and years) more than the JD plastic bush did.
If you have to replace the tractors drive belt, Engine to tranny pully, it is best done when you have the tractor stripped, as its not an easy task, as a lot of the stuff on the bottom of the tractor has to come off just to thread the belt to where it has to go, so raising the tractors front end almost straight up or tilting it on the side is necessary for ease of ding this replacement.
The bottoms of the front spindles axles also wear more so thats another area to check. The disk brake on this model are a little on the weak side, and it only takes a little bit of oil or crud on the small 3" diam or so disk that the brake pad rides on, so a new brake disk puck is cheap and easy to replace, and worth while as long as you have the tractor stripped down. Its located on the right side of the transaxle right near the portion where the drive axle come out of the tansaxle case. You may also find this seal in this area to be worn, which will also cause oil from the transaxle to get on the disk rotor and pad.
I hgave had to replace the front wheels a few times, as I did a lot of cutting on an angle, and it seemed like there was too much sideways pressure on the area of the front wheels where they are wleded to the steel tube that holds the wheels bearings, and they used to crack and come apart right at the weld. Only had this happen to the front wheels not the rear.
Pull t he fuel tank and replace the insulating pads that isolate it from chaffing on the operators deck and frame, as dirt and just normal deterioration will allow dirt and usch to chaff the tank wearing a hole in it.
The fuel level sending unit in the tank is prone to pin holes and will keep the fuel level light on. I replaced mine 2 times, and got tired of spending $60 some bucks it every time, so I did without.
These are about the only areas I have ever had problems with in the many years I owned my 180. Once I changed the spindles bearings and removed the inner seals, all my spindle bearing problems wet away.
What exactly was wrong with this tractor that it was at the point of not being uneconomical to repair? Engine, Transaxle or what? The 170 / 180 tractors are pretty much bullet proof if they were given just a small reasonable amount of care in their life.