100Mbit ISDN solution

When faced with a similar request recently, I found nothing
in the low-end market that had a 100BT interface.

You can get one on a Cisco 36xx router.

Or for less money, run from your Pipeline into a switch that has
at least one each 10BT and 100BT ports. With an ISDN link, the
10BT "bottleneck" is a moot point.