Option 1: Clean out some data. Find out WHAT is s#^&#^&#^&#^&#^&g up all of your information, and trim it. This will reduce your backup times and keep your existing system going longer. BUTTTTTT!!!!!! There is a finite timeline on how long tapes will last and tape drives. They will fail. 5 years is in that fail range. Price: Cheap - time to rumage through to see what can be eliminated or trimmed by a system professional
Option 2: A 5 year old DLT will take a LONG time to back all that stuff up. A new LTO-2 will backup your 50GB in 1.5-2 hours with verification. The tapes capacity is in the couple hundred gig range. This is the traditional way, and for ease, I would do a full nightly with Monday through Thursday tapes, Friday 2-5, and Friday 1 Odd Month, Friday 1 Even Month. This will give you some individual file recovery based on date over extended periods of time. Even better is having the ability to retire a monthly tape but that can get costly. Price: Couple grand
Option 2, look into a DATTO. I am installing these at several clients in the next month or two.
http://www.dattobackup.com/ This will allow a full data backup onsite AND offsite. Because we know someone religiously takes a tape offsite for disaster recovery
Price $300-800 initial outlay, $200 - 400 per year. ASSUMES you have decent internet speeds.
Ohhh and next time they are looking to do servers, you guys should get a Small Business Server - perfect for your size organization....