When it comes to using Express Checkout you just need to have a valid set of API credentials. From all of the info on PayPal that I have seen you should be able to get the API credentials for using Express Checkout for free. It is only if you need to make use of Payments Pro with the Direct Pay service that you would need to sign up for Payments Pro and pay the monthly fees for it.
I have also heard from others that having a business account is what costs a monthly charge. This is what I meant in my last response about signing up for the service where I said that you should check with PayPal on this.
If you have the API credentials you should make use of them for an Express Checkout only type of checkout.