Total newbie here and I'm looking for some guidance from the experts! We have booked a river cruise next May, starting in AMS, ending in BUD. Searching flights out of BOS and finding Lufthansa schedules (transferring FRA) that look good. I am a MileagePlus member but with no significant mile balance so I'm not wedded to them (41K). Looking to book Business class.

Question: Do I book through the United site or Lufthansa? Pros/Cons? I assume I will earn miles if I book through United so that seems to make sense. However, when I look at the itinerary on the UA site, it looks like I might not be able to reserve a seat which is important to me. I see this message -

"This flight is operated by a partner airline. Seat selection for this flight may be available on united.com after completing your purchase."

So the selection "may be available"? What if it's not?

Also - is there a rule of thumb for how far out to start looking at flights for international travel? We won't leave until the middle of May.

Thanks for your help. I have booked a ton of domestic flights but I am really inexperienced with international. I appreciate any advice.