I'm glad that you are aware that you need to consult a lawyer, because you do. The way language is read in the law is not always the same it is read elsewhere.
From the looks of the EPL you should have all rights to modify and redistribute what they provide you, and they have no right to a royalty payment. Section 2(a) of the EPL states:
Subject to the terms of this Agreement, each Contributor hereby grants Recipient a non-exclusive, worldwide, royalty-free copyright license to reproduce, prepare derivative works of, publicly display, publicly perform, distribute and sublicense the Contribution of such Contributor, if any, and such derivative works, in source code and object code form.
However, Section 4 looks like it puts a responsibility on you to potentially have to defend anyone involved in the software that my get sued as a result of their work on the software.
Now you might run into trouble if what the other party is contributing to your software is governed by a separate licensing agreement. Section 1(b)(ii) states:
where such changes and/or additions to the Program originate from and are distributed by that particular Contributor. A Contribution 'originates' from a Contributor if it was added to the Program by such Contributor itself or anyone acting on such Contributor's behalf. Contributions do not include additions to the Program which: (i) are separate modules of software distributed in conjunction with the Program under their own license agreement, and (ii) are not derivative works of the Program.
So if they are acting on your behalf, which it sounds like, and they are not giving you something that is a separate module of software distributed in conjunction with the program under its own licensing agreement, you should be good.
Again, to be 100% sure you need to consult with an attorney, as there are all sorts of likely exceptions to the EPL, or other open-source type licenses, that you must be aware to fully understand your liabilities and rights.
