I'd buy used. I bought a used ford brush hog that was from the 60s, built like a tank. I paid $350 for it because it was pretty rusted and looked bad, but works fine.
Is there a safety clutch of some kind if you run into something it can't cut, like maybe a rock or small stump or does that just need to be avoided?
Most have a slip clutch, better to try to avoid things. If the clutch slips much you have to retension the springs, if the disks are worn you have to replace them.
/ Just bought L35, what kind of mower (flail, brush hog, etc.) to get?
You're correct, you need to watch what you run over or you'll end up pulling off hydraulic hoses or electrical lines. You may need to go backwards the first time to make sure everything is down to a reasonable height.
Also, never used a 3 point hitch...will the unit pivot up, or lift up while still level?
There needs to be flexibility in the top link or you will be stressing the hog's frame when you go over uneven ground. Some people use a chain for the top link, that allows the rear wheel to ride up and down. There are also toggle connections that allow that motion. But don't leave a rigid toplink in place while you mow or you can bend the unit. I disconnect my toplink because I never transport it, I just mow with it.