Hardtail vs Dual (FSR) is a decision YOU have to make. But there are some items you have to consider. You're on the east coast so riding over roots is a fact of life. Dual suspension is way better at handling trails with root problems. Also, you're an older rider, (but I've still got you beat), and dual suspension is way more forgiving on the back and neck compared to a hardtail.
However, hardtails make you a better mountain biker because you have to pick your path more carefully. IMHO, similar level hardtails are faster, due to less weight to throw around. I have both a hardtail Stumpy and a dual sus Stumpy. On smoother trails, I take the hardtail and let it rip as much as I can. On a trail I'm not familiar with or a trail with lots of roots, rocks, logs etc. I take the dually to get through all that stuff.
As for a used bike, hard to say. I went used with both Stumpys, but I knew EXACTLY what I wanted. I think if I were in your situation, I'd stay with new.