This is a rather hard set of criteria to meet. As written above there is essentially only one well supported DSL-SoC Lantiq's xrx200 and that is getting long in the tooth for anything but pure bridged modem duties (now at your ADSL speed you might get away with not noting that unless you start to install and use CPU intensive packages).
In your shoes I would probably try to get my hands on a recent FB 7520 and try to install the work-in-progress build discussed in:
No idea about how well this does ADSL (newer DSL modems tend to get optimized for VDSL duty so ADSL might be limping along) but at least the CPUs should not hold you back too badly. But that is neither a stable built and I have no experience how easy it is to install in the first place (plus "cheap" for a 7520/7530 is still a bunch of EUR).