1. R' Moshe (o.c. 187) has a great diyuk from rashi who lists among the items that require hidur mitzvah - לולב נאה, טלית נאה, ציצית נאה. What is a "talis na'eh"? Is there a kiyum of hidur for one to purchase expensive clothing? R' Moshe suggests that it is referring to what we call a talis - a garment meyuchad for tefillah. From this he proves that even a minhag that is for the sake of kiddush hashem, such as a tallis that one davens in, requires hidur mitzvah. Based on this he says that the din hidur mitzvah is not limited to chiyuvim, but also applies to things that one has no obligation in and is only there for kiddush hashem purposes.
2. It is not clear from our gemara how much one must spend on a positive mitzvah (see my sefer). Tosafos proves that one certainly does not have to spend all their money, and the gemara implies that one doesn't even have to spend a third of their assets. The Rama in hilchos succah assumes that the limit is a fifth, which he learns form tzedaka. However, Maharatz Chiyus quotes form Mor U'ketziah (ya'avetz)that if we force someone to fulfill a mitzvas aseh עד שתצא נפשו, one's money should not be more important than their physical pain, so one should be obligated to spend all their money. The maharatz chiyus himself only challenges this from the Ramban who limits the din of כופין אותו עד שתצא נפשו to one who rebels against the entire mitzvah. However, most rishonim don't learn like the Ramban, so the question of the ya'avetz would return. How can it be that one does not need to spend all their money, but you beat him until he dies? To me it seems that the answer is obvious. The din of כופין אותו is not a punishment, rather a way to get him to do a mitzvah that he is required to do. But, when the mitzvah cost more than a fifth of his assets, he is not obligated in the mitzvah. The beating is not equivalent to spending, because the beating applies only when he is obligated and not performing, but the spending limit determines whether he is obligated altogether. See Tosafos Menachos 41a where there is an implication that כופין אותו עד שתצא נפשו is not only a push to do the mitzvah, but is actually a punishment.