Turning an in-house project into a cloud-ready multi-tenant app?

I might have a side-project coming my way. These guys own a business and have had an outside firm develop a business-critical software for them over the last couple of years (ASP.NET MVC 5, MS SQL). Now these guys would like to share their expertise with the world and turn this tailor-made solution into a SaaS. I have yet to talk to them in-person, but I have a feeling they do not want to rewrite the thing, but simply do the necessary changes to make the app multi-tenant.

What should I be aware when going into this? Personally, this looks fraught with danger for multiple reasons:

• how the hell do you even do such a rewrite? This seems like a requirement that touches on all parts of the code?
• unknown codebase of possibly substantial size. I have not seen the code itself, so I cannot judge the quality. I have seen a bunch of screenshots (of over 90 forms), so I am judging the app is not small.
• unreachable original authors
• most of the logic is in stored procedures – cannot unit test stuff.
• several hand-written components instead of adopting industry-wide solutions (ORM, authorization)
• maintenance – who is gonna be in charge, me? Original creators?

I have been developing enterprise apps for a number of years now, but this is my first side-project, so I don't know what to be aware off.

• What should I ask these guys project-wise?
• Do we discuss money upfront or only after I determine if I am interested in doing it at all?
• Should I recommend a total rewrite? If a total rewrite is off the table, what would be my next best option? Skip the project altogether? -> This is the most important point for me.