As with all homes, you need to ask for the market comp's in that area - "like-for-like". That will tell you what price you should be paying for that property.
If this agent is going to represent both of you, they have a "fidicuiary" obligation to represent both of you professionally & ethically so both parties benefit. Make sure you feel comfortable with the transaction before you sign any contracts.
the phrasing of your question indicates you saw the home through a listing agent, not a buyer agent. If this is the case, did they disclose to you that their fiduciary responsibility is with the seller, not you? Of course they'd like you to buy the home, but it is now their job to get the best terms for the seller not for you. For instance, if this home is going for above what other comps reflect it is his duty to try and justify and defend that price... whereas a buyer agent should be using that as ammo to negotiate a lower price. More importantly, if there is a BETTER deal out there... this agent would never advise you to get what would be a better home and/or deal, where a buyer agent is agnostic about the home you buy. It does alwasy amaze me that buyers continue to go directly to the sellers representative instead of using their own, which typically costs nothing.
In NJ, the listing agent who enters a dual agency situation by representing the buyer as well, is required to step back from the transaction. The agent needs to become neutral and not give any confidential information to the buyer or seller without permission. The agent really can't advice much in the way of negotiations.
That being said, there are definitely agents who know how to walk that fine line and serve both buyer and seller just fine.
Know that the listing agent will receive double commission (and if it is the managing broker the ENTIRE commission because there will be no broker/agent split). What it comes down to is how much confidence do you have in this agent to do what is best for YOU?