A Gen1 TC is both too small and too old to base a lot on. It could die at any moment.
You can bridge it and plug into the new TC.
You can use it to extend wireless network.
These are no cost options for as long as the TC lasts.
There is no such thing as load balancing on a TC.. and it is very complex anyway.. you need a dual wan router.
A second adsl line can be used for redundancy for sure. The TC is just router in this situation and you need to buy a new adsl router so I doubt it is much use planning around the TC.
Thank you for that. How does bridging work in this case? If there's an article I can read that'd be great.
How to extend apple wireless networks.
I'm wondering what speed advantage I'll get from a new TC. Even my new macbook pro doesn't support 802.11ac.
Some people get major improvement due to the better antennas.. some people get none.. Suck and See.