Unless you already have usable components you're planning on upgrading, the ordering sequence doesn't matter, since you won't have a rideable bike until you finish. The timeline for accumulating stuff becomes more important if it stretches out beyond a year since things keep changing and you'll have to worry about compatibility. If you're only talking about a few months or less then let opportunities and sales drive the sequence.
One thing you should think about is the total component cost for your new bike. If you can find a complete bike with specs similar to what you want, it might be cheaper than the components bought separately, and you'll have a frame to either sell, keep as a spare, or build up with chaep or used stuff into a "B" bike.
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