Creating a network link between 2 very close buildings

A wireless access point/router in the building with DSL, combined with a wireless access point functioning as a repeater in the second building, should be sufficient.

You are right that using WEP is out of the question. However, many contemporary access points will function properly as a bridge or repeater on a network that is secured properly with WPA2. You may also be able to squeeze additional functionality into existing hardware by upgrading the manufacturer's firmware to DD-WRT. (Check the DD-WRT web site to see whether your hardware is supported.) You are most likely to have success with this if your wireless access points use a Broadcom (not Atheros) chipset.