The problem that I see in your question is this phrase: "my husband and I are wanting another baby and feel that we are at a point where the older two would be helpful".
That's not a good point to factor in when deciding to have another baby. You and your husband would be the parents and the ones to raise the baby, and just having older kids doesn't mean you have built-in helpers or babysitters, although certainly older kids can be expected to help with all household responsibilities. However, caring for an infant when a pre-teen is 11 is not at all the same as raking the leaves and loading the dishwasher.
Sure, tweens are always excited, about new babies, new puppies, a new bike, a new family car, a trip to a theme park, even the thought of getting an extra large pizza AND cinnamon sticks AND a 2 liter bottle of soda !!! But they don't have the maturity to consider the accompanying responsibilities and tasks: cleaning up afterwards, staying home to babysit, saving up to pay for the tickets, keeping the back seat of the new car free of Cheetos, gathering up the pizza boxes and taking them to the recycling bin, paying for the gas or the food, storing the bike properly every night, paying for the vet bills, hearing the baby cry, knowing that mom and dad can't come to middle school events without the stroller/backpack/diaper bag, etc. "Hooray! We're getting a boat!" is quickly followed by "what do you mean we have to scrub it and change the oil and pack the food for the day and then unpack the food at the end of the day and clean the barnacles off and hose down the deck and repair the sails and coil the lines just so? I thought we were going fishing."
So, if you really want another baby, I suggest you remove the other two kids from the equation. Do you want to be solely responsible for an infant and then a toddler at your kids' dance recitals, middle school activities, sports events, high school parents' nights, etc, without expecting help from your older kids? Your older kids might be involved in practices, rehearsals, sleep-overs, scouting, summer camp, study groups, sports, homework, and all the other things that require time and effort, either at home or away from home.
Just some things to think about...