No going FF is not the best way to get started. You need an interchangeable lens camera and ideally for macro a mirrorless camera. Mirrorless cameras have two big advantages that matter for macro: First focus peaking. You can enlarge the viewfinder image for critical focus. Second you can adapt just about any lens.
Yes, Yes and Yes. 1) You don't need FF. 2) If you like the idea of live view and the ability to see what you will get, being able to zoom, and other abilities, add that. 3) Any lens! You are going to be shooting manual, Macro isn't fast, there could be stacking, you may want to bracket the exposure.
I think if the OP is looking for a new/used camera to move forward, a mirrorless should be the first consideration. The brand doesn't matter, but if you like Nikon, fine go with that. There are a number of models and manufacturers mirrorless cameras, now on the market, which have become discontinued by updates and upgrades. The sensors are often the same or just as suitable, with less cost.
After how much do you want to spend, I'd ask, what are you going to shoot? Macro is a pretty large and broad term. Macro what? If the answer is not specific, just fun and diving in, that's a great answer also. Marco for the sake of experimenting and experiencing, is a challenge and entertaining.
How macro do you want to get and how much fun do you want to have. Many options. Oh and these are a 50-D that I have on a stand. Single frames, just having some fun.