Besides the usual services like Stripe etc. which most will likely recommend, I would highly suggest you look at http://fairpixels.pro these guys are the best I know of when it comes to helping companies design beautiful b2b saas UX/UI.

Depends on what exactly you need. Ideally you should be focusing on building only the core of your product yourself and outsourcing the rest to other service providers. Ex: billing + invoicing to a reseller, hosting to an IaaS service etc.

If you are asking about specific development tools/libraries, again I suppose it would depend on what exactly you are building.

I think you should use the tools/services whichever gets your job done the fastest. First get the MVP out their. Beauty comes later.