Many —I'd perhaps even stretch to "most"— processors have at least the basic functionality like email receipts and, to at least some degree, recurring payments. They're staples of e-commerce.
But yes, there is work involved in implementing each provider. Stripe has some but it is generally nicer to use. £100k of volume and you'll probably pay for that additional development time in Y1.
But also consider that you shouldn't irrevocably tie yourself to a single processor. Don't rely on them to do your business for you. Sometimes they're dicks. I know that applies more when we talk about PayPal and much less in the "real" merchant processing world, but it still happens. Having the ability to switch out processors is objectively useful so (eg) tying your basket UI into Stripe's checkout js lib looks nice, is super easy... But what happens if they start withholding funds? If all your emails are tied into their system, it's weeks of development to stop using them. Do business-critical stuff yourself and you're much more flexible to do sensible things in the future.
Regarding international rates, these are hard to compare because their claims are apples to oranges. Stripe say they go off their "banking partners'" daily rates and that those are approximately 2% above mid-market levels. Paydoo says they're 2.49% fixed above market levels. I suspect they're pretty close in practice. Stripe does let you accept multiple currencies (for free) if you have the bank accounts for it though. Not sure if Paydoo allow the same. Their wording suggests not... But if that's the deal-breaker, talk to them about it.
