You state that she is only your girl friend. That means that she can do what she wants with the land and you are out of the picture.
Before I would spend 300,000 Baht on a girl friend I would want to have some sort of guarantee that I could get some return on my money should the relationship head south.
If you want to spend that kind of money giving your girl friend a gift - why not go all the way and have your marriage registered. That way you won't be 100% out in the cold in the future. You will have the ability to recoup some of your investment.
Trust me - over here it isn't "just money." You have no idea of what will happen in your future and you might just well need that money yourself at some time in the future.
If you have 300,000 you want to just toss down the drain - which essentially you could be doing - how about loaning it to me? I can possibly pay you back, with interest in a couple, three, four years.
Your intentions are good. But to me - you are setting yourself up for major problems. The family is going to be turning to you to set it up for farming and what not. You could be setting yourself up to being seen as a "cash cow."
As for the price - 16,666 a Rai is not a bad price at all. Make sure that the land has a clear and clean title to it. Make sure that it wasn't government land to begin with - as this could cause problems in the future. Know of one case where it had been government land, but the family selling the land "squatted" on it for a number of years and that resulted in a major court case when the village wanted the land to make a park (supposedly.) The family lost the court case - but it was only after the intervention of the Thaksin government that the court case was over ruled and the family could then legally sell the land after certain conditions had been met.