Your characterization of the E5640 sounds right - a little more powerful and less energy efficient.
I don't understand the recommendation for a 5 amp fuse - I'd look at the factory specs and run whatever Dell recommends.
A hardware RAID controller may or may not be useful. First, you generally don't use a hardware RAID controller with unRAID in RAID mode. Instead, it is useful if it can be flashed into IT or JBOD mode, passing individual disks through to unRAID. unRAID is a software RAID implementation that needs access to the individual disks. Some hardware RAID controllers can be used this way, some can't - depends on the model. There are rare cases where you'd actually use a hardware RAID controller in RAID mode, but I'd recommend saving that discussion for another day. By the way - you definitely should run unRAID on a UPS.
You are correct that the parity drive needs to be as large as, or larger than, the biggest data disk. You're also correct that 2TB drives aren't a great buy right now... You can copy from one disk share to another disk share, though, if you want to do a two part migration. There are some rules about this - check back if you go that route. There's no reason to use a big flash drive, 4GB or 8GB is fine.