I'm not asking for complete idiot-proofing
How big are your unrars? I think you need to set the disk free space limit to maybe 2-4 times your typical unrar size. The default is 1 GB and clearly that's not enough.
The problem you're having right now is that unRAR and download are both consuming space at the same time and colliding with each other. UnRAR checks and notes it has enough free space to unrar, the download continues, the two processes consume disk space and eventually you run out because, once the unrar starts it keeps going to completion because it already checked and it HAD enough free space. The free space just went away because the download continued.
Subsequent unrars will stall because when they check if there's enough free space, they'll notice there isn't enough. So, you need the download to stop not when the disk gets close to empty but when the disk free space gets close to a multiple of the UnRAR size.
[Filename]
MinFreeSpace = X
In the configuration file, how about setting X to 20000 and see how the downloads work out. If your typical unrars are 50 gigs, set it to 100000.
If you're downloading and unraring to a drive that's not being checked for free space, only the unrar free space check works and as I've pointed out. It's not sufficient if your download speed is fast enough. In that case, you're better off downloading to the main download path and then unraring to this other drive.