PEK to Beijing South should take under an hour at that time on a Sunday. Allow double that on a Friday afternoon! Price will be around 100RMB and probably more like 85RMB - this includes the 10RMB toll for the airport expressway and a couple of RMB that gets added on in some taxis for fuel surcharges (so the fare may be up to 12-13 RMB more than the meter shows).

Alas the station will be chaotic - it always is! The journey is short enough though, and you should be able to get a ticket for a departure without waiting too long (First tends to sell out first a lot of the time).

Have you considered booking a car to Tianjin? There are various limo companies around - e-hi is one of the bigger ones and they have an English booking service (http://corporate.1hai.cn/english/ ). You can also email them in English. More expensive than the train of course, but less hassle if you're coming from PEK and cheap enough compared to what you would pay for a limo in Europe or the US.

Last option - though not especially luxurious - would be to take a coach service from PEK to Tianjin. You are at the mercy of traffic and your fellow passengers, but it is at least simple and will be cheap. Buses go from outside the terminal.