By the way, for all the punters... I ended up using Fetchapp. It works quite well. Apparently hosting a downloadable sellable SU model on your own site is a complicated HTML coding exercise, so I didn't go down that path. Even more baffling is trying to set up the unfathomable Paypal Digital Downloads account. (Paypal need to lift their game with the user-interface of their products. For a company of that size,.. what they give you to work with is just woeful)
Anyway Fetchapp, different story, very easy to use. They host your model for $5 a month. No other fees. You can easily create a standard Paypal Button that links your Paypal account to your website. FetchApp provide the HTML code, you copy and past it into your website program (I used Muse). I'll let you know if it works if I ever sell anything π