Couple of other things to think about. With ISDN you also get voice phone service and the system is able to switch between data and voice so when phone is not in use get 128kbps when phone is being used drops to 64kbps. Factor that in when thinking about cost.
posted be skeptical of claims abut massive increase in speed due to compression. That only works if what you are downloading is not already compressed. Since you are a gamer latency is important, compression adds latency so you may want to forgo it for that reason alone. ISDN being end-to-end digital has very low latency.
Getting 128kbps out of ISDN requires bonding the two connection, the ISP may change extra for that.
You may want to talk to your ISP and see if they offer IDSL. This is basically DSL but using ISDN signalling. Speed is slightly higher because there is no voice, 144kbps and you are not making a dialup connection.
You can share any connection, I shared dialup on our home LAN for many years. However now a days web sites are optimized for much higher speed and graphics intensive browsing can be a challenge at such a low speed.