Danilo is right, you would want to have your own agent represent you. In this instance, the Realtor for the investor would not have your best interest at hand. The terms broker and realtor are used interchangeably, however, the broker is who the realtor works for. For example, I am with Long & Foster. I am the realtor, Long & Foster is my broker. I handle all of the transaction acting as a representative of my broker. My guess as to why he would want you to not get your own realtor is that he would not have to pay the other half of the commission. His agent would probably give him a discount. He would save about 2-3% of the sales price at your expense. It sounds like a really great deal, except did you have an agent show you what the comparables are for the neighborhood? That is a huge neighborhood and if you click on the link below, you can see what the homes in the last 60 days have sold for. There are also homes for sale starting at $325K. If you choose to purchase, even from the builder, definitely use a realtor to help you. The commission is built in and you will be better informed. Good Luck
If the seller has a problem with you having your own representation, they may be trying to hide something. It's obviously not that they don't like Realtors because they have one representing them. So it's just you that they don't want represented.
The list of what to be cognitive of is huge and is why Realtors still add value to the process. Get your own Realtor/Buyer's Agent or hire an attorney in order to protect yourself and your $300K.