The easiest way to get to Ban Huai Yang is to fly by Bangkok, alternately Phuket, from there you can go further with domestic aviation to Hua Hin or go by bus, train or cab directly to Ban Huai Yang. Flying to Thailand can be a bit expensive so the cheapest way is to take a last minute flight during off season. Once here everything is very cheap. A cab from Bangkok to Huai Yang, approx 200 miles, is around 80 euros if you for example go with Nung cab service that is based in Ban Huai Yang (contact info in the right column). Bus the same distance is approx 3 euros and train between 1 and 5. If you take the bus or train try getting help at a tourist office they get you the ticket and gives you a ride to the station. For this they take a little extra but not much and it's worth it because it can be a bit tricky to do yourself. Take the opportunity to that stay a couple of days in Bangkok, it is an enormously exciting city, hotel rooms are cheap and there is often a couples of nights included in the price of the flight.
For shorter distances in the cities I suggest to go by tuktuk, kind of a taxi moped, it's extremely cheap and quite nice. The tuktuk drivers are masters of short cuts over sidewalks and through alleys and avoids the traffic jams that you easily get caught in with a regular taxi, especially in Bangkok. They are also very helpful but they can have a tendency to drive you to places you don't want to go to, since they receives provision from certain shops, clubs and restaurants. But if that happens just ask them to continue to your destination.
The trains go both north and south from the station in Huai Yang every day. The highway from Bangkok, that holds a very high standard, goes immediately outside and continues south. From there the buses depart up to 25 times every day. Cab is easy to get a hold of in the village and the blue local buses stops if you wave at them. Everywhere there is water there are boats and they are often happy to give you a ride for a small fee. In the village you can rent scooters, mopeds and cycles. Mind the left hand traffic!
Outside Prachuap Khiri Khan there is an airport where you can rent an eight seated private plane that flies you within Thailand.