I think you already answered your own question. Start with the market you know - investment banking.

This is clearly an extension of virtual document rooms. Start there. Who buys VDRs? Target them. Figure out which conferences Merrill and Intralinks attend. Go there as an attendee, corner a few people, and figure out who else has the same problem. Sell a few of them. Then buy a booth at the next one and set up some real meetings.

One huge trick to SaaS is to stay very focused on an achievable, small market first. Then, as you get traction, start expanding you markets and capabilities. The biggest problem in SaaS is that either your development team or your sales team is always going to be behind. It's much more likely it'll be your development team. The more markets you add just creates more initial complexity and slows down your developers.

Build a tightly defined product and market it to one market. Then create derivative products and expand your brand.

I'm happy to chat through more strategy with you if you'd like to go deeper, but I think you already have the first market figured out.