NY to Florida Keys. How difficult will this trip be?

I've made this trip many times (although not for a number of years), and have never found it to be very difficult. That being said, you'll want to make sure you have all the spare parts you'll need for routine repairs along the way- there aren't always contractors available.
Study the charts, follow the markers, and there are plenty of really nice stops to make. If you're planning to make the whole trip inside, it can be a LONG one because of all the no-wake zones.
A search of this forum should give you all the info you'll need. There have been plenty of discussions over the years. My personal favorites are Annapolis, Charleston, Savannah, and St. Augustine.

Belts, filters, hoses/clamps, raw water impellers, pumps, toilet parts. Tough to say more without knowing the boat. Imagine the things that could really ruin your cruise if they fail, and have spares for those things.

Are there still cruiser guides available and chart books?

Waterway Guide and Cruisers Net also have information. Active Captain still very useful. Then other resources to look at what is on land.

However, the "how difficult" question throws me. Why would it be difficult? Only one reason, Captain. It all boils down to you, your skills, your experience. That is what determines the difficulty.

While I greatly prefer running outside, there are a couple of areas I definitely wouldn't run inside. NJ and south Georgia.

Given your post and comments, your only off shore run is all of New Jersey.
Given good weather, it’s not bad .
Bad weather and it’s hell.
Let the weather be your guide.
From Cape May, I would run inside down Delaware Bay to the Chesapeake all the way to Florida till I got the Fernandina and then consider running outside.
Sounds like you have time in your side .
Forget about going out and around Cape Hatteras etc.
Plenty of anchorages and marinas on your way down to stop at along with help and repairs if needed. And generally much calmer waters.

It’s not a race!

Yep!!

I love the trip and agree with others… NO to NJ. Also, I’ll never do GA segment of ICW again. Mud river was super shallow and stressful but I do like St. Simons

Have you done some short trips in your area? It's just a series of short day trips. Having the right mindset is critical. There is no race, you can run for a few hours and tie up or drop anchor to enjoy a new spot. Pick your weather and go outside if the weather, available inlets and the distances are favorable. Talk to anyone on the dock each afternoon and find out what they think about the area you're going to run through the next day and write some notes on your charts if there are concerns. Enjoy!

We just finished part of this run. Chesapeake to Jacksonville and back. All inside with 46 sedan bridge w/4’ draft. The guidance mentioned earlier is provided by a sailor who has mapped a recommended ICW track that is crowd source updated regularly. Search for bob423 online to get more info. Also, we used both our Garmin chart plotter along with an iPad running Aquachart. Aquachart gives you a detailed, colorized view of depths in the ICW based on USACE data. It’s a very economical tool. There are some skinny areas in GA but as mentioned earlier the tides run 6-8’ so timing is key. Only additional comment is if you are running at speed (20 kts+) be very focused. There are areas where you can be out of the channel in the blink of an eye.

I made the run from Ft. Pierce to Southport late last month. 37' Luhrs drafting about 3 feet. The inside run from Ft. Pierce to St. Augustine was fine, except for a couple of long NWZ for manatees. After you make the crossover at Haulover Canal (heading north), there is a LONG NWZ for manatees near Edgewater, FL. Also bridge timing can be important. My outriggers are 31.5 feet, so we had to open a few bridges and wait for a few others to open on schedule. We ran outside from St. Augustine to Charleston and back inside from Charleston to Southport. Again, there are a few bridges on the inside run.

Bob's Tracks (bob423) are invaluable when running inside. He's a sailboater with a deep draft, so tracks are more important. There are a few spots in SC, Georgia and Florida (particularly between Jacksonville and Fernandina Beach) that I would have been more concerned about without updated tracks. My transducer was working intermittently (and always failing in skinny waters), so I relied on the tracks at times. I used them with Aqua Maps on the iPad and iPhone. The app is free and Bob's Tracks are free. There is a small charge if you want to update the Aqua Maps zones you are traveling through (I think it was $2.99). For what it's worth, our Raymarine MFDs with updated Navionics charts also provided similar track information.

As for stops, we enjoyed Charleston, St. Simons and Fernandina Beach on the way down and St. Augustine on the way home.