Does Wix include a spellcheck feature for text boxes?
What I mean is a user facing spell check for users who need to write longer bits of text.
No, the text element does not come with a user facing spell checker. This is functionality you would need to build yourself if it is required.
To save you time building a feature like this, would you be able to point users to a browser extension to install if they wish? Something like grammarly?
Here is the documentation for the text element if you would like to read through: https://www.wix.com/velo/reference/$w/text/introduction