So - if you've got past experience of these features, and can therefore estimate your time and resource need well, and know what major features are Must have (M

So - if you've got past experience of these features, and can therefore estimate your time and resource need well, and know what major features are Must have (MoSCoW is your friend), this in my mind strengthens the case for waterfall - as you can accurately (enough) plan out a sane project plan which includes your must haves and has slots for your Should and Coulds which can get pushed out in contingency, and you can then run the whole thing with predictability, predictable budgets, predictable feature sets, and not end up caught short in a mad sprint cycle at the end. Agile is great if you've got a weak scope, or an unlimited budget, or all the time in the world - but in the scenario you describe, waterfall would be the model I'd apply.