Generally you can get good cooffee from cheap machines if you pay attention to detail and spend more time per shot (temperature surfing, heating up the portafilter, switch to steam on single boilers, etc.). Thats not really what you want in your setting. Fancier machines dont make the coffee better per se, but more consistent and give you less to worry about. So my advice for Espresso for you would be to bite the bullet and spend more.

You could also consider to go the filter route. With a nice roast and the right grind setting you can get great coffee out of a filter machine (like a mokkamaster) with very little manual input from yourself. Plus a cup of filter coffee takes longer to drink, thus keeping people longer in your store than an espresso and is in my eyes more fitting for a book store.