Your OW flight will arrive in Terminal 2, LH will depart from Terminal 1. So besides immigration and customs, there'll be a terminal transfer. Then you'll have to clear security in terminal 1. Additionally, if travelling with checked baggage, you'll have to collect your bags in T2 and re-check them in T1.
Taking all that together, and considering you'll be travelling on two different tickets, I'd leave at least three hours between flights to be on the save side.
A o/w eco classic ticket FRA-TXL, which includes a checked bag and a seat reservation, will typcially cost you 79€ (or a bit more during peak times). The same ticket in C will cost at least 309€.
The upgrade is not worth it IMO for a 50 min flight. You get the same exact seat as in eco. Hard-product wise, the only difference is that the middle seat is kept free (the planes are configured 3-3, but in business they'll only put 4 pax per row). The meal you'll get in C is nothing special, and drinks are free in both Y and C. IMO, the only arguments for booking C would be (i) lounge access, (ii) security fast lane, and (iii) more flexible booking conditions. But, ofc, you'd get (i) and (ii) anyway with *G status.
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I second the other poster's comments on taking the train from BUD. It's a ten-hour train ride. Typically, you'll take a regional/slower train to Vienna (so you may enjoy the view) and then transfer to an express/ICE train from there. The ICE train takes six hours and runs at speeds of up to 150 mph, that's often times too fast to enjoy the view. Besides, it runs through tunnels and along anti-noise barriers which block the view most of the time.
Last edited by 1flyer; Sep 17, 2018 at 10:41 pm