What you want, is their clients and their LTV.
As you do not care for the software at all ( and those features I guess you will have to re-code them ), ask them to buy the customers.
You go ahead and pay them, the LTV of the client for a year, if the user re-new or want to move over to your platform. The ways you can do that are many, but into my eyes, it is a certain way to manage know, how many of those total 2.500 leads and 1.300 customers will keep paying for you. Because if you pay more than should and never re-coup, no effort.
Also, do you want to buy the brand?
What is their domain value, backlinks, any content etc?