Rather that starting at the launch end of the mission, I look at the mission from the end and work backwards towards the launch. In this case, I'll just examine the Earth-Moon transfer vehicle. A similar analysis would be necessary for the lander vehicle.

1. Consider what it would take in terms of structure, propellant, etc., to slow the returning vehicle from about 25,000 MPH down to LEO orbital velocity of about 17,500 MPH. The goal of this step is to determine the starting and ending mass of the vehicle during this entire phase.

Once you've done that, work backwards.

2. How much propellant would be required to accelerate that mass from lunar orbit velocity (approximately 4,000 MPH) to the return to Earth velocity of roughly 6,000 MPH. Include the necessary reserves of propellant for mid-course corrections and the mass of the consumables used by the crew on the 3 day return trip. Add that mass to the mass calculated in step 1.

3. Next, determine the amount of propellant and consumables used while the vehicle is in lunar orbit. Again, add this mass to the result from step 2.

4. Next, determine how much propellant would be slow the vehicle from the arrival velocity at the moon (approximately 6,000 MPH) to the lunar orbit velocity of about 4,000 MPH. With no atmosphere at the moon, it has to be done using thrusters. This requrires a lot of fuel. Add that mass to the result from step 3. If the vehicle will include a payload intended for lunar orbit or the surface, you have to factor that in as well when calculating the amount of propellant necessary to slow the vehicle.

5. Determine how much propellant would be required to accelerate the mass from step 4 from LEO velocity to trans-lunar injection velocity (approximately 25,000 MPH). Before you do that, add in a propellant margin for possible mid-course corrections and don't forget the consumables used during the mission.

The key point of all this is that it takes propellant to accelerate propellant. For every pound increase in vehicle mass at the end of the mission might require several pounds of additional propellant at the start of the flight.

By analyzing the flight in reverse, you have your best opportunity to determine what the starting mass for the mission will be. Once you've determined that, then determining which launch vehicles to use becomes pretty easy.