It's not mandatory in NJ to have a signed written agreement but it is required that agents inform the prospective client of agency relationships verbally and in writing by giving them a copy of Consumer Information Statement (CIS) prior to any substantive discussion of the prospect's needs and wants. You can certainly limit the scope of the agent's service to you if you don't want to end up stuck with one agent. The agreement should allow you to specify the timeframe of the agreement, whether for specific property or type of property (i.e. multi-families vs. single, or commercial), and location. It also specifies how much your agent would get paid if a deal closes, but if the seller's broker offers less than the amount on your agreement (say, 2.5% instead of 3% on your agreement), you're liable to pay him the difference.
The purpose of having a broker buyer agreement is to clarify the agency relationship, i.e. whom does the agent work for. It stems from the fact that many times buyers would go directly to the listing agent thinking that because the agent acts all friendly and helpful, that he's now "their agent," and start to tell him all sorts of secrets about their buying needs, how high they could go, etc. Not knowing that the listing agent's fiduciary duty is to protect the seller's interest. Then the buyer gets taken advantage of because they end up saying too much. Having your own buyer's agent is supposed to give the buyer a level playing field.
However, you should still limit the buyer relationship so you don't end up oweing a commission if you signed too broad an agreement and ended up working with another buyer agent, the first agent might have a claim against you. Read the contract and only sign when you both agree to the terms. A smart agent will try to get it to be as broad and as long as possible. It's up to you to be the smart investor to make the terms favorable to you. You can just as easily make the contract on a per-property basis or do a month or so at a time.