In this, as in many other sentences, I think that the simple present should be accepted as well.
The present progressive is needed here, because a person can buy his fifteenth house only once. The simple present is for a repeated or usual action.
Why don't you accept numbers?
You are here to learn the language. Not numbers.
You sound like my Latin teacher 30 years (thirty) ago. Thank you :)
I think it should be ok to write numbers in English but to force letters in Czech
I definitely agree - allow 1st, 2nd, 3rd, 10th, 15th etc.