Selecting a time slot and date is only required for entry to the Acropolis if you wish to visit at a specific time/date, due to the potential crowds and strict limits on the number of visitors per day.
For all other sites, you can purchase your tickets directly at the ticket office; you won't encounter long queues, or even any queue at all.
Hadrian's Library is a small site next to Monastiraki Square in the Roman Agora area. You can see it from the outside without even entering the site.
Many sites, the Acropolis, the Acropolis Museum, both Agoras, Hadrian's Library, Kerameikos, and several other museums are located in the same area within easy walking distance. Once you've decided on a time slot for the Acropolis, organize your other visits as you wish, without stressing about following a precise schedule.
The Parliament building with the changing of the guard, the National Garden, the small "Cycladic" Anafiotika neighborhood, Syntagma Square, Hadrian's Gate, the Benaki Museum of Greek Culture, the Cycladic Art Museum, and many other places are also located nearby, less than a 15-minute walk away.
Or would it be best to hire a guide for the area?
A more detailed, informative, and personalized private tour can be arranged with an official guide, the only ones authorized to enter the sites with you.
A list of guides is available on the official website of licensed guides.
Here is a list of English-speaking licensed tourist guides in the Athens area with their contact information:
https://www.tourist-guides.gr/guides/list-of-licensed-tourist-guides/?bla=1338&q=attica