・・・、？
The prep work's mostly done... how about taking a nap?
Literal
Prep [topic-は] mostly finished-[and so-し]..., one-sleep do-[honorific いらっしゃる]-[if-たら] how [copula-です][question-か]?
仕込み in a kitchen or restaurant context refers to mise en place — advance preparation of ingredients. The trailing ~し attached after a polite verb (終わりましたし) is the enumerative 'and' particle left dangling — 'and (so, the natural next step follows)...' いらした is the honorific auxiliary いらっしゃる conjugated to past, attached to 一眠りして to honorify the whole 'take a nap' verb: 一眠りしていらしたら = 'if you were to take a nap (honorific).' Combined with どうですか this forms a deeply polite suggestion to a superior.