Welcome! I'm not sure if you'd be able to import 2 -> 1, unless you import them as separate companies within Blesta. If you'd like to give it a try, PM me your trial key (Settings > System > General > License Key) and I'll add an addon company to your trial.
If they are in separate companies, they'd be in the same install, and you can switch back and forth between companies to manage without having to log into a different location.
Re: Stripe, probably not, I believe they just released that. Give it a spin. It may be something that you would need to migrate manually. Blesta uses "Payment Accounts" for CC and ACH, so depending how they store it it may be imported, but may need some finessing to get working.
I'd say, give it a spin, and we'll help with any questions or issues that arise. https://docs.blesta.com/display/user/Migrating+to+Blesta (Shows WHMCS 5.x, but has been known to work with 6, not 100% sure re v7)