Is the 4210 a MFWD, and what is the transmission, standard or E-hydro? I've owned a 4210 for 3 years and 240 hours, if you have any general questions about the model feel free to PM me. You could also read my review of my JD.
Land Pride attachments are an excellent quality and the 65% price is fair since they are so new, just be sure they are an item you would need and use. Since he likely got a 10% discount or more off list, what you are asked to pay is about 50% of list which I'd happily pay in your situation.
I'd guesstimate that a new 4210 equipped with the FEL and 72 inch MMM, assuming it has MFWD and E-hydro, should cost around $21000 delivered, depending on dealer markup where you live. Good luck !!