There are some good thoughts in some of the comments, but I don't think that you should pick the option to pursue - the Owner should. Develop a proposal that outlines all 3 options, describing your role, approach, and limitations of liability for each one, and the associated fee. There is a value to your coordination skills, so you should put a price on those skills based on how much time it will take you to to complete the work. If you think it will be less effort on your part to coordinate your own MEPFP team, your **** should reflect that, and may make that option the most cost effective. This is as simple as it sounds: frame the proposal as Option 1, 2, 3, describe what you will do and what the Owner (and Owner's separate contractors) are responsible for, and the associated fee for each option. I don't know the Owner's priorities as well as you, but you can tailor a multi-option proposal to suit their needs and make sure that you get paid appropriately for each option and that your risk is mitigated accordingly.
While I agree with Youssef Matta's opening statement that you should explain the pros and cons of each approach, I would not blindly recommend any percentage of MEP design fee for your services. You may have no control or knowledge of the quality of their deliverables and/or what fee value is going to be presented to you. 20% is a shot in the dark. Ideally you could just charge a straight hourly rate + reimbursables for this service, but if you are going to be held to a fixed fee, you need to protect yourself with a conservative fee. Oddly enough, this could be the opportunity to explain risk protection to the Owner and let them know that if you don't know what you're getting into AND you need to submit a lump-sum fee, it will necessarily be high.
These can be hard conversations to have with Owners, but I've found that that the "three options" fee proposal opens the door to talk about what the Owner is getting in each scenario and can help introduce transparency into your fee structure as well as give you the chance to speak to your skills and the value you bring. Sometimes the one fee-one description with disclaimers can feel oppositional to an Owner.
------------------------------
David Mentzer AIA
Dore + Whittier Architects, Inc.
------------------------------