What are your overall goals for this land?
[The 10 acre big field will one day be pasture. When I bought it, it was loaded with Johnson Grass (a horsey no no) so I am having to revamp it for now. It is also a 'buffer' between my home and the adjoining land.]
What do you want this place to be? Without knowing that, it's hard to provide good advice. On the face of it, you have too many tractors, because you seem to only need a big lawn mower. But as I said, what are your goals for the land?
[For the rest of the land, it is pasture for my horses.]
Why are't you using the 10 acres for grazing or hay?
[For grazing I addressed above-it is currently not suitable for horses. I contacted local hay growers and the cost benefit was not in my favor. I do not have the time or experience to grow hay on my own (or the equipment needed to bale, etc]
I raise cattle and mowing grass pasture for appearance sakes seems wasteful to me. You say that it's very hilly and that's another good reason to "hire" some livestock to graze it for you. It's safer and easier then mowing and if the stock belongs to someone else, there's no additional labor for you. Are there neighbors who might lease pasture from you? Have you considered increasing your stocking rates and thereby reducing or eliminating the cost and time to mow? There are several ways to manage this land to generate some income.
[I would love to generate income but the pasture is not currently fully fenced. I also ride on that field so having cattle around could be...well...interesting if I took my crew over there! LOL]
That machinery sitting there is costing you money in terms of depreciation and deterioration. It's not good to let equipment sit unused. Figure out what you want to do with the land and then you'll know what to keep, sell, and/or replace.