We will be day tripping from Munich to Fussen to visit the castles on Oct. 27th. Will we be ok purchasing the train tickets in Munich the same day of travel ?
No problem. We're not talking about an airline, where you have to reserve weeks in advance. Riding the regional trains in Germany is more like riding the city bus in your town. Use a Bayern-Ticket (23€ for the 1st person, 4€ each additional up to 5 total). It will cover your roundtrip train from Füssen (after 9 AM workdays) and the bus between Füssen Bahnhof and the bus stop in Hohenschwangau.
Yes, buy a "Bayern Ticket", if you leave after 9 am weekdays or anytime weekends it is 23 euro for the first person and 4 euro for each additional person up to 5 total traveling together. That will cover you there and back. Buy from a station vending machine or pay 2 euro more from the ticket window. Info here.
If you are using a Bayern-Ticket, you don't strictly have to wait until after 9 AM on workdays to travel. The first train to Füssen after 9 AM leaves the Hauptbahnhof at 9:53 and gets you to Füssen Bhf at 11:54 and to the Hohenschwangau bus stop at 12:13. If you want to arrive an hour earlier, there is a train leaving the Hauptbahnhof at 8:53 and leaving it's first stop, Pasing, still in the inner zone of Munich, leaves at 9:00 AM. You can use the Bayern-Ticket from Pasing to Füssen.
A one zone MVV ticket for 2,60€ will get you to Pasing on the 8:53 train. If you are staying somewhere else in the inner zone of Munich and need to take public transportation to the Hbf, that same one zone ticket, for 2,60€, will also be valid for the 8:53 train to Pasing, from which you can use the Bayern-Ticket.
If you can wait until 9:53, or it's a weekend, buy the Bayern-Ticket the night before (make sure it is for "tomorrow" when you buy it), and you can use it (after 9 AM) to get to the Hbf, without having to have a separate MVV ticket in the morning.
The others have provided great advice on the trains, but don't forget that you'll require time-specific reservations to visit Neuschwanstein.
"The others have provided great advice on the trains, but don't forget that you'll require time-specific reservations to visit Neuschwanstein."
Those aren't necessary to buy in advanced in October. At that time of year, you can usually show up without a prior reservation by mid-day, and you rarely have to wait longer than an hour after buying your ticket.
Thanks for all the great advice.
Given the Current DB rail strike, are there other options for trains from Munich to Fussen ?
See my response under German Rail Strike.