To the OP, not that I would go this direction; but a 10 or 15 year old, 2wd, 30-50hp, gear tractor, open station, without a loader might be found for 6-12k; and would still be miles ahead of what your working with. Lot of times school boards, park departments, ect have these for mowing ball fields, and they might have 1000-2000 hours, but there isn't a lot of demand for them, so it pushes the price down.
Edit: some of the 70s, 80s and early 90s tractors can be hard to get parts for, so just keep that in mind. That's one thing an 8N or MF 35/135 had going for it. Heck autozone probably has everything that you can't find at a local hardware store.
It depends on exactly what '70s/'80s/'90s tractor you get. Most compacts of that era had production contracted out by the company that sold them, the only major company that sold much in the U.S. then that didn't do that was Kubota. Parts availability on rebranded third-party tractors is often pretty poor that far out from when they were made. Full-sized utility tractors of that era have pretty good parts availability for the most part as they were made by the companies that sold them and often the designs were used for years with few modifications. But, these will be at the "50 HP" end of that spectrum rather than the "30 HP" end.
Isn't there a brand or two out there that has figured out how to meet the emissions standards without resorting to DEF and regen cycles?
DEF is used on >75 HP units as the regulations for emissions tightens at >75 HP, and as far as I can tell, nobody has found out how to meet the regulations at >75 HP without the catalytic reduction (SCR) that DEF is used to accomplish. I am not aware of any unit 75 HP and less using DEF as it is not needed to meet the 75 HP and below regs.
The 26-75 HP units all have an oxidation catalyst (DOC) to meet the various non-particulate emissions regs (hydrocarbons, nitrogen oxides, etc.). Some use EGR and high EGTs to keep the catalyst face hot enough to burn off soot so they don't need a particulate filter (DPF) to meet particulate emissions regulations. This works unless the engine isn't run long enough or hard enough to get EGTs high enough to burn off the soot, in which case you get catalyst face plugging and the DOC has to be removed and cleaned, or replaced if it can't be cleaned. EGR has its own issues too, it cruds up the intake, reduces the amount of oxygen in the charge air (which decreases power), and either increases the temperature of the charge air (which decreases power) or requires an EGR cooler to cool the recirculated exhaust. EGR coolers can plug up or fail and cause head gasket failures, just ask anybody who owned a truck with the Navistar VT365 engine, aka the "6.oh-no," about that one.
If they do not want to use EGR or use EGR but do not want to keep EGTs persistently high, they use a DPF in front of the catalyst to trap the soot and then use a temporary increase in EGTs is used to burn off the trapped soot (regeneration.) The no EGR setup gets rid of the issues with EGR and EGR coolers, and having an engine setting that can clean out the DPF eliminates the face plugging issue, but requires some user intervention in relation to the DPF regeneration. Otherwise, the DPF will get plugged up enough that the regeneration cycle won't burn off the soot and it will require removal and cleaning, or replacement if it can't be cleaned.
There is no free lunch when dealing with the feds, unfortunately, only different kinds of bad.