Travel agents in India are relatively cheap. They can take a lot of the headache out of bookings, especially if you're taking a bus or train or if you're staying at a local hotel. For air travel and international hotels probably not needed. Booking domestic flights within India are cheaper vs booking from abroad. So a travel agent could help with that as well.

You don't need to book Taj tickets ahead of time. You can simply walk up to the ticket counter and purchase it. There are separate prices for foreigners vs Indians/NRIs. But as pointed out, the train to Agra gets filled quickly. You might consider hiring a car/driver to take you around. The downside is the terrible traffic.

Varanasi can be overwhelming, especially to first time travelers. It is a densely populated city with loads of temples along the river banks. Cows are everywhere as it is a sacred city. Be prepared for the smells and sights. Definitely get a guide. The daily aarti on the river bank is not to be missed -- highly suggest going to Dashashwamedh Ghat.