First of all, thanks to all the extremely helpful people who answered.

If it matters, I get there on the 20th of June, which is a Wednesday. After reading the responses, I'm feeling more optimistic about my chances of making the earlier train than I was going into this whole thing.

Fanjet's point was exactly right about looking first for the long-distance train and then worrying about getting to the main Hbf on time. I should have started that way!

largeeyes' point about the bottom floor at the main HBF is useful, so thanks for that, I'll keep a lookout for where I leave from. david7031, a huge thank you for the info on the size and usual wait at customs/immigration. I recently went on a trip where I waited almost two hours just for immigration (Bangkok - it was awful!), so it's a relief to hear that something like that isn't likely. Very interesting to hear your thoughts about taxis vs RE train as well.

So, here's what I'm thinking, after processing all your input. I'm thinking of buying the tickets for the 19:48 train from Berlin HBF to Bielefeld. Flight arrives at 17:10, in an ideal world we'll be through baggage claim/customs/immigration and walking out of the airport by 18:15.

If that happens, I have an hour and a half to get to the HBF, so I'll just walk over and get on the RE. Do I have to buy a ticket ahead of time? I'm assuming I can get one on the platform.

If we're delayed somewhere along the line, then we'll have to plan on the fly. I'm guessing the thing to do if, say, I don't get out of the airport until 19:00 is to grab the first taxi we spot and try to get there on time. Alternately if there happens to be a train leaving within 10 minutes, we could do that, but I'm suspecting that if it's a longer wait than that it will be faster in the taxi. I'd rather avoid the taxi if I can because my budget is limited and that's 40 E I'd rather not spend, but if it means missing the train or catching it then obviously it's worth it.

Thanks again to everyone who posted thoughts!