There is no financial reason to buy Toledo train tickets in advance (unlike many other Spanish rail tickets, whose prices tend to increase as the travel day approaches), but trains can sell out--especially the latest trains back to Madrid. Rick specifically suggests buying the return ticket at the same time as the outbound one, so you don't risk being marooned in Toledo when your hotel room is in Madrid. Important: I just checked fares for next week, and the round-trip is cheaper than the one-way fare, so make your best guess about how long you want to stay (I spent 3 nights there), and buy the round-trip ticket.
Unless you're traveling around the time of a holiday, I think you can safely wait to buy the tickets from a machine once you arrive in Spain. If you decide to make the purchase in advance, you can try to use Renfe and pay via PayPal; US credit cards are apparently a problem on that website. Otherwise, go with trainline.eu.
The tourist office in Toledo sells a little bracelet that gets you into 6 or 7 secondary sights (not including the cathedral, the El Greco Museum or the Santa Cruz Museum), with a bilingual tour offered twice a day (as of 2016) at most of them. I loved those sights. It's also wonderful just to wander around the back streets of the very large historic district. Toledo is a bit like Venice in that visitors tend to be concentrated along the path between the usual arrival point (Zocodover Square) and the cathedral. That area can be a zoo, and the cathedral is popular enough that you may need to pick up a timed ticket and wander off for awhile until you can get inside. (This can be a bit of an issue if you're trying to catch the tours at the bracelet sights.) Walk a couple of blocks away and you'll be in medieval Spain. It is really wonderful.