An excused person who performs wudu before prayer can keep on his/her worship until the wudu is broken by a reason other than the reason which causes the excuse. They do not need to change their clothes or underwear even though urine, blood, faeces, pus, vaginal secretion, etc. smears on them during the prayer, as long as the state of excuse continues. Yet, if these unclean substances shall not contact the clothes again, they need to be cleaned.
The Arabic word “imam = إمَام ” means “leader”. The imam is the leader of that congregation. Leaders, in all matters, are chosen from among the wisest, most knowledgeable, healthiest and most qualified of the people in a community.
A person who is excused of wudu can not lead the congregation of healthy people, because wudu is a pre-requisite of the prayer. However, not all the excuses prevent becoming an imam.
For example, a blind person who meets all the prerequisites of prayer, including wudu and the cleanliness (taharat) of his body and clothes, can lead the congregation. It is known that Prophet Muhammad (pbuh) used to assign Abd-Allah ibn Umm-Maktum frequently as the imam of the congregation while he was out of Madinah. Abd-Allah ibn Umm-Maktum was blind by-birth. Even though he had an excuse, he was a knowledgeable Companion trusted by Prophet Muhammad (pbuh). In the narration of al-Barraa ibn Aazib, he said: “the first one to come to us of the Muhaajireen (in Makkah) was Mus’ab ibn Umair and then after that it was the blind man, Ibn Umm Maktoom. Ibn Umm Maktoom was one of the two principle mu’athhans (callers to prayer), of the Prophet (SAWS), the other was Bilal. Sometimes Bilal would call the athaan and ibn Umm Maktoom would call the iqaamah and then other times it was the other way around.