As said, I would recommend asking for developing a new authorize.net SIM payment gatway (not starting from our authorize.net AIM payment gateway).
If you implement E-check into AIM, you will still be subject to (each couple of years more stringent) PCI-DSS compliance and audits.
Authorize.net SIM, now refered as "Payment Form" has an integration guide here:
developer.authorize.net/api/upgrade_guide/
It supports both credit-cards and E-checks, and does not require PCI-DSS audits of your server and business.
There is a developer who is not affiliated with us that develops CBSubs payment gateways. You could certainly ask for a quote for a complete Authorize.net SIM gateway with credit-cards and E-check support (and possibly any payment method SIM supports, as it's easy since it's all hosted at authorize.net):
www.joomla-payment-gateways.osdcs.com/index.php?option=com_digistore&controller=digistoreProducts&task=list&cid
[]=12&Itemid=52
This is not a recommendation, as I didn't review nor security-reviewed their gateways, but as you asked, that's one that develops CBSubs gatways.
Hope this helps
Please let us know in the future here if you decided to go with him and how it went.