Hi Fredrik,
first thing I thought after reading "forties (Fahrenheit)" was: What about summer heat down there? Living in southern Germany, we had a lot of days around 32-35° C this summer and it was no fun to run.
Now, up to your questions:
I can't recommend treadmills fitting to your needs by name, but I can tell you some experiences wich I made on our fire stations. We have to kinds of treadmills here:
The ones you know from hotels etc wich have a rubber belt and can display running videos and change grade according to those. I used it sometimes but didn't find the videos fitting to my speed. Even, the grade adjustment was not fitting to the video, running steep uphills in the visualization and doing it without the grade adjusting, felt weird. I think there was a threshold wich short uphills didn't meet.
The other ones are sturdier, don't have a rubber belt but planks and can be used in two ways: powered and self propelled. I prefer them a lot: they feel better when running on them. I use it most times in powered mode because I can chat in that mode more easier with my coworkers and we watch TV while doing it. Just set your speed and give it a go. If im doing a structured workout, I either type in every step when the step is due or I use the self propelled mode, wich is more close to running outside. But in that mode, you need to concentrate more on your pace because it only depends on you. Those mills can submit speed via BT, but I can't use that with the forerunner. But it could be used with every Zwift capable device.
All treadmills I ever encountered where able to receive HR via ANT+, this should be an easy must have.
What I do on the treadmills: I put an iPad in front of me, connected to the FR945 via Virtual Run mode and running Zwift. This gives me some video to look at and all the data I want when running. Cadence, Pace, speed, HR. To get accurate data onto my FR945, I use the RUNN sensor. This transmits speed and cadence via BT and ANT+ and will make every inaccurate Treadmill to an accurate one wich connects to the watch (I want it on the watch recorded because of training load and recovery time). If you only want to have the data inside Garmin connect and don't care about FirstBeat algorithms, you can tell Zwift to write its file into GC. Also, there is a ConnectIQ Datafield wich collects grade from the RUNN and stores it together wich elevation gained into Developer fields in the .fit file. That way it is documented, but not really used by Garmin Connect analytics. I use some 3M velcro tapes to connect the sensor to the mills, that way I can change the mill the fastest way if one is already occupied or I have to change the station. The stickers on the belt are overseen by most and the rest doesn't care.
åI would recommend that way for you. You can buy any mill and connect it fully into Garmins ecosystem without paying a boatload of money for ANT+ broadcast. It means you only have to buy one where you can program the suggested workout and dont have to think about accuracy. Zwift running is free of charge right now, there aren't enough to monetize it yet.
In Central Florida, it appears that our high temperatures are approximately the same as yours. I don't recall ever breaking 100 F (37 C). The problem I have is that it's constant for four or five months. It will drop below 80 F (26 C) most days after midnight, and start climbing quickly once the sun rises. And the humidity usually only drops if a hurricane or tropical storm blows through. And that's only a couple days of dryer air before it all comes back. The end result for me is that I'm running at 5 in the morning, in the dark. I really dislike getting out of bed earlier than I need to.
Thank you very much for the suggestion about the RUNN sensor. That would definitely make the treadmill's data much easier to deal with.
Nordictrack do treadmills that automatically adjust incline when using their ifit program, you have to manually adjust pace though. We have an older model that does this but don't really use it much as we have Zwift and runn set up on the other treadmill.