Is the Eucharist invalid if the priest is living in serious sin or an immoral lifestyle?

No. The validity of a sacrament depends on matter, form, and intention, not on the personal holiness of the minister. This was defined against the Donatists in the fourth century. The Church teaches that the sacraments work ex opere operato, by the act performed, because Christ Himself is the true minister. A priest in mortal sin commits sacrilege, but his sin does not contaminate the sacrament. God protects His people from being deprived of grace due to the unworthiness of His ministers. The Eucharist confected by an unworthy priest is the true Body and Blood of Christ. The priest’s guilt lies in his conscience and will be judged accordingly, but the faithful receive Christ Himself.