"Google" Ken Diamond travel in Hua Hin. I've used them for the past nine years on our trips to Hua Hin. They are a Hua Hin based travel company with a very good reputation.

They will send a driver at no cost to you from Hua Hin to pick you up anwhere in Bangkok. They use Volvos and Mercedes. The drivers are skilled and safe (although the last one we had drove at around 150kph, although not dangerously!).
That trip took 2.25 hours, but usually allow 2.5+ hours.

The cost was $2500 baht each way which is well worth it door to door. In Hua Hin you will find their office just down the soi from the Hilton Hotel.