we are thinking of using a font available on Typekit Marketplace on our website. This seems to be a one-time cost to "purchase" the font from Typekit Marketplace and as long as my Typekit subscription (actually it's Adobe CC) there will be no additional cost?
This isn't completely clear to me from the information available.
Thank you for your interest in Typekit! The Marketplace purchase is a one-time licensing fee to add the font to your account.
In addition to that, however, you may need a paid subscription to host them via Typekit. Marketplace fonts can be hosted from any subscription level, including Typekit Free, but the subscription pageview limit still applies. For Typekit Portfolio, that is 500,000 pageviews/month.
If you site receives more traffic than that, you would need to upgrade to the appropriate higher-level paid plan to support your web font traffic:
I hope that this helps; let me know if you have any other questions.