If you're splashing out on MB then a $49 certificate from EV1 won't hurt.
Customers will appreciate the fact that they're securely transmitting their personal details on your sign up form (as a web user I generally don't offer my personal details unless it's encrypted with SSL).
SSL inspires confidence - it'll pay for itself in the long run
I agree...even if you won't be putting the clients payment info through MBill, it is still a good idea to secure modernbill itself with an SSL. It will look much better to your customers if you have your order/billing software behind an SSL certificate. A little piece of mind (although most GP have no clue what a SSL cert actually does...they just look for the lock and are happy) goes a long way.