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I don't have a Trimble fm750, but from what others have posted, yes, you can input RTK corrections into it from a CORS system. Yes, the fm750 has to be unlocked for RTK. A few different ways to get cellular/internet based corrections into one:
1. Bluetooth to serial adapter and an Android smartphone or cellular capable tablet running an NTRIP client:
2. Aman Enterprises has a relatively pricey dongle to do a similar thing, but works with iPhones and cellular iPads.
Now being in the hills of eastern Iowa and if driving across the slope, you may not be super happy with an EZ Steer. It will probably still drive better than most can by hand, but I've heard the NavII used with integrated steering (using a hydraulic valve) compensate better for being on a sideslope. EZ steers do that too (have gyros for roll), but maybe not as well. My fields are in Wright County, prairie pot-hole country, so most is pretty flat. Of course, you could get one used (see them pretty often in classifieds here or on eBay; probably on facebook marketplace as well) and try it out. You probably can resell a used one for what you bought it for. Probably going to spend $2,000 to $2,500 on a used one? Haven't been paying attention lately on current used prices.
As to options for "better", that is a tough call. I've been happy with Trimble 500s with EZ Steers, probably been using for close to 10 years already (and they were going out of production then). Drives a 1000 times better than I ever did by hand when I started. For the $, its been a great deal. I'm sure integrated steering works "better", less wiggle down the line, holds a curve a little better, less stuff in the cab, etc, but can be quite a bit more. For some/many, that is worth it. As to the 750 itself and options of keeping, unlocking to RTK, getting used 750 w/ RTK unlocked already, newer Trimble, or switching to Ag Leader. That gets complicated. Depends on budget willing to spend and what you end goal is for other uses (planter clutch control, sprayer, move to combine, etc). A 750 can be readily used for planter clutch control and sprayer control. I use one of my Trimble 500s w/ an EZ Boom for my pull type sprayer (rate and boom control), don't have row clutches on my planter but it could be used for that as well.