EPA requires lower emissions for engines over 25hp. Most manufactures achieve this with a DPF or diesel particulate filter. The filter typically has some rare metals that not only filter but help to clean the exhaust. Another less common option is DEF which is diesel exhaust fluid that is injected to help lower emissions. The third option, and the one Mahindra brags about, is a catalytic converter. The advantage of the converter over the DEF is the DEF requires a regen every so often. The regen requires higher than idle RPM and injects fuel into the exhaust to heat the exhaust and burn off/clean the DPF. The regen lasts about 20'. Time will tell which is better.....
If you are looking at new tractors old posts would have you avoid new because of early problems with the conversion. You also see companies like Kubota and others offer tractors right at the 25hp mark to avoid the emission requirements. The emissions equipment adds a few thousand to the cost.
Now it seams most have accepted the new reality and are willing to purchase the right machine for them even if that means it has emissions.
Another change that has come with the emissions requirements is computer controlled engines. No opinion offered as you are asking for information at this time.
If you are planning to avoid the emissions and buy a pre emissions used tractor good luck! I was willing to travel a few thousand miles and looked nearly daily for 9 months. Lots of folks are thinking the same thing! The machines I found were very close in price (with the cost of transport) to new ones or hammered.
Almost 2 years ago I bought a 1988 Ford 1920 FEL 4x4 (shuttle shift) from a Craigslist advertisement.
Tractor was in very good condition, but not excellent, with 1056 hours. It did cost me $1250 to have it delivered 420 miles.
I rebuilt the 3pt hitch (sloppy), replaced clutch (it did not need a clutch, but I WANTED to do it), new seat, new water pump, thermostat, & hoses, new battery, all fresh fluids, and new filters, adjusted valves, had bucket rebuilt (much stronger), new rear rims & tires w/tubes, (CaCl removed, WWF now!), sheet metal sandblasted or sanded, entire tractor primed and painted.
The tractor looks like new, and runs like new.
No DEF, no plastic parts, no catalytic converter, no electronic controlled anything.
I have less than 2/3 total dollars invested as compared to a new 32HP 4x4 FEL, but ..... most important .....
I have a great little 32HP 4x4 FEL tractor that is simple & fool proof, and will likely NEVER need to see a dealer!