There are all kinds of tape drives to look at. I would suggest going to the Dantz web site
http://www.dantz.com and first and foremost make sure any drive you are considering is compatible with Retrospect. If it's not, it's not worth having IMHO.
There has been a lot of talk lately about
DDS 3, DDS4 and Ecrix. I use DDS 3 at the office because of the low cost of drives and tapes.
Others might favor another. Basically, you will have to decide for yourself. I think Ecrix has a 30-day money-back-guarantee right now, so you might try it and see if it does what you want it to.
Good luck.