Any tractor is going to need the radiator/screens cleaned if you bush-hog for a day in the right/wrong conditions. Breaking a PTO shaft was most likely because it was binding because the drawbar wasn't set right. It's not a big job to fix a timing cover leak, you will pay that bill anyway because the dealer will deduct enough off the trade price to cover it. Front drive hub is "one of those things", if you have MFWD tractors you will most likely fix one...careful use and obsessive checking of oil will minimize those problems. Engine speed sensor should not have been many $$$.

You can trade often enough to always be in warranty, and never have a repair bill, but that would be about the highest possible cost hours on a tractor.

It's your money, do what you want.....I believe you would be way money ahead by keeping this unit. From what you have described, I would not label it as a lemon.