Some things for you to consider in your search:
1) Don't expect too much in the speed area for whatever you buy. RAID5 isn't going to be breaking any speed records.
2) Do some research on the chipset the device's RAID controller is using to make sure it's legitimate.
3) Personally I don't think i'd buy any hardware RAID device with less than a three year warranty. If the device fails and you can't find an identical replacement then you're basically screwed.
4) Can't stress the importance of using a UPS in conjunction with these things enough.
Finally, I think I'd explore other options if I were you. For starters you may not need time machine for the entire system. The advantage to time machine over other "cloning" backup schemes is versioning. If most of your data is movies/pictures, that probably isn't something you need, unless you find yourself casually deleting your files on a regular basis. Versioning is going to be more useful for files you're constantly changing.
I'd consider options like data archival and drive mirroring. If have a 1TB drive with all your documents, music, user data, and applications, then yeah, back that up with Time machine to an internal or external 1-1.5TB drive. If your 1.5TB drive is full of pictures and videos, then just buy another 1.5 or 2TB drive (or two, to have some added redundancy, maybe even in a RAID-1 array) and create a mirrored backup.
More information about your data usage would be needed to formulate a solid backup strategy. But my point is that RAID 5 may not be necessary for you. If your total data consumption is less than 3TB, then I can almost promise that it's not. If you can avoid using it, then I would advise you to look into cheaper, simpler, and perhaps even more reliable options for your backup solution.