Additional payment gateways and modifiability?

I built a crowdfunding platform for a client of mine similar to your fundraising plugin, but they keep coming back for additiona l functionality, so I'm considering just replacing it with your plugin (which has many of the changes they want), but have a couple questions.

- Do you guys have any additional payment gateways available for your Fundraising plugin (specifically Stripe)?
- I haven't worked with any of your plugins just yet (but I wish I had a year or two back when I was doing some MU sites!), how easy to modify core functionality is it? Are there generally hooks that I can modify functionality with, or would I be screwing around with the core for most changes?