I had one of these in my Spyder -
https://www.t7design.co.uk/2-2kw-mic...9-cc4-d35.html
It never really put much heat out, I'm afraid, even taking into account that it was in a completely open car. I've gone for a much bigger heater matrix from a production car (VW Corrado) with a surface mount fan for my latest project, which is also going to be completely open.
As for fresh air inlet and demisting, the two really go hand in hand and I'd say they will be essential for safety reasons if you get caught in a downpour with the hood up.