Wow, that certainly provoked some debate!
I have looked at Barrie Williams' follow-up article today, and it suggests that the detail of Brampton's instructions is not known. (Though there may be subsequent scholarship of which I am unaware.) Apparently his information rests chiefly on Portuguese sources.
Susan is right about the general use of women as pawns in marriage; however in this particular case Joanna was mad keen to be a nun and had actually turned down no less than Maximilian of Austria (future Emperor!) and the Duke of Orleans (effective heir to France after Charles VIII). As these two were arguably as important as Richard, it's at least interesting that Richard was accepted, and maybe even more surprising if the initiative came from the Portuguese side. Apparently Joanna accepted on the basis that if the proposal fell through she would be allowed to take up religion, and so it transpired. This does not prove in itself that Richard was a saint - this was part of international diplomacy after all - but it does give food for thought.
Apparently the 'alternative option' under consideration was the eldest daughter of Ferdinand and Isabella of Spain. From Richard's point of view an attractive aspect of either woman was that she had a Lancastrian claim to the throne, arguably more legitimate than that of Henry Tudor - certainly it would have been a senior claim by 21st century calculation of these things. Richard had some very positive diplomatic contact with Isabella right at the start of his reign, so it might have been another possibility.