1. you can easily setup an unraid with 1 data and 1 parity. I have no idea what format your existing drives are in so I can't answer your data migration question but more likely leave them where they are and migrate the data over the network.
2. pre-clear happens in the background so you DONT have to take unraid offline to add a disk.
3. can windows read unraid drives. no, not natively and I dont know of reiserFS IFS for windows but it might exist but unlikely.
4. There are tons of downsides to unraid just as there are lots upsides. Which server solution will works best for you is all a function of what your priorities are. you certainly can use a UPNP server with unraid though without the need for another machine. Whether its acheivable based on your level of experience or tolerance for exploration and time spent is another question all together.
unraid can share via SMB and also NFS although I find NFS support weak. AFP is also supported in 5.0
4. I'm pretty tech savy, and dont mind the time in exploration, but i know essentially nothing about Linux (isn't that the base for UnRaid?) How difficult is it to set up UPnP on UnRAID? Are there instructions out there?