Need to take the train to Freiburg after flight lands in Frankfurt. How much difficulty and inconvenience, if any, should be expected? Also,after trip, will need to stay one night at an inn or hostel in Frankfurt before taking flight out next day, any suggestions?
Assuming cost is not a major objective and you want to get their quickly, go out to the Fernbahnhof (long distance train station). There is an access road in front of the terminals and a pedestrian bridge across the road from the 2nd floor mezzanine of Terminal 1. Just look for signs to the "Fernbahnhof". Go across the bridge and out the hallway to the station. There you can catch express trains to all over Germany. There is a ticket counter on the upper floor. Turn left when you get to the station.
As for a place to stay before returning, I always recommend Hotel Koenigshof in Mainz. It's reasonable priced and right across the street from the Mainz Hauptbahnhof. From there it is a short (30 min) S-Bahn ride into the airport.
I have tons of suggestions for Frankfurt, tell me your budget. Also your dates, to see if coincides with any big trade fairs. These make hotel rooms difficult to find and pricey.
There is plenty to see here, so make sure you set aside a bit of time for sightseeing in Frankfurt.
The bahn.de site shows that there are direct trains from the airport (Frankfurt Flughafen) to Freiburg as well as departure times that require one change in either Mannheim or Karlsruhe. In any case. the standard 2nd class fare is 61 EUR.
If you book well in advance (up to 90 days allowed), you can get a Dauer-Spezial fare as low as 29 EUR. Those fares are for a specific departure date and time and are non-exchangeable and non-refundable. You print the ticket yourself. If you want to take advantage of the discount, then you'll need to allow lots of time between the time your plane is scheduled to arrive and the time your train is scheduled to depart. Otherwise, if your plane is late, causing you to miss your train, you'll eat the price of the ticket.
If it's important that you catch the first available train after your plane lands, then book a full fare ticket online and print the ticket yourself. It will be good on any train and you won't have to allow time to buy a ticket at the airport train station.