Hi @Michiel
Welcome! You will certainly find a range of opinions here on Webflow hosting prices - but for me I can relax knowing it’s some of the best, fastest, most stable out there - and it’s seamless with the tool itself - no FTP work needed.
Some users prefer to export their code and FTP it - I prefer to keep it in the same place, knowing I can reach out to the same support team as I would with any Designer or account issues.
In terms of exporting the code, you will need a plan that allows it, however you can upgrade/downgrade your ‘Designer’ plan at any time. support@webflow.com can advise further on that option.
Availability calendars - you could look at these two posts for ideas:
E-commerce - you can find lots of posts about that on the forum for sure!
Webflow has of course recently announced they are working on it:
You can embed Shopify buy buttons for example, and there are other examples like Ecwid and Plasso working with Webflow - again search away - there is also some further detail on integrations here: Webflow Integration resources | Webflow University
Myself and many others have worked with @foxy to make online stores with Webflow - the beauty of their integration is that you/the client can manage the products inside of Webflow, rather than managing listings in two systems. Foxy have a page for Webflow, and offer support above and beyond the call of duty 
Webflow | Foxy.io
Hope those are useful!