What is the most straight-forward way to simulate this circuit?
I think just using the loading currents and voltage values for the datasheets of each component, add them all together should give you a power consumption within the error you specified, if nothing is broken just doing that should be a decent approximation. For each resistance just use the current/voltage that the datasheet specifies for the component is connected to.

Now if you have to simulate it and have no models, again for the error you said 50% just making dependent sources(current or voltage) with the values from the datasheet should be enough.

Making new models from the datasheet parameters should be entirely possible, sometimes the datasheet itself includes either a description or a link on how to SPICE model the component so just looking at that might be worthwhile.

Honestly, I don't think you need to go as far as simulating to get a power consumption within 50% of this circuit if you have the datasheet parameters for each IC.