Don't buy a dell, stupid purchase. Why buy a piece of hardware that you are going to have to wait for days to get replacement parts or worse, not being able to get them.
We repair computers from Dell/HP/Gateway/etc all the time, and 99% of the time you can't get parts from them because something is out of stock or out of production. Not to mention that they rape you on the price. Then because the case is generally proprietary in some fashion, you can't just buy something off the shelf and insert it. So, that means the customer is stuck buying several different components just to replace one.
Get a PC built locally out of standard parts that you can buy anywhere, or quickly online. You'll save money in the long run, and your client won't have to tolerate downtime.
As for your backup, tapes can be a pain, same with other types of removeable media. I would setup a mirrored SME box, and then maybe use one of the windows boxes to do a secondary backup to itself.
BTW, you are in for a world of headaches with those Access databases. Especially if they are big and complex. Access is crap that crashes and corrupts data.