Audi A8 consultation

Hello and may God's peace and mercy be upon you. Regarding the 2013 to 2015 models, Which engine is the most reliable in terms of fewer breakdowns? Are spare parts readily available, or does availability depend on the region? Are the costs of regular maintenance high, or are they reasonable? Comparable to American or Japanese car maintenance. Note that usage will be 80% or more within the city, And I generally take good care of my vehicles. I'm currently driving a 2013 Taurus and am interested in an Audi from this category. I'd appreciate advice from anyone who has used or tested these models. May God reward you with goodness.