You are currently shipping dozens or hundreds of boards per week from a catalog of dozens or more designs that rev frequently
You should get a high end pick and place machine. It will be a money printing machine1 for you. It will let you keep only a small number of any SKU on hand and be able to react quickly to spikes in demand. You will also be able to rev your boards as often as necessary without having to worry about retooling or obsolete inventory costs.
This is the AdaFruit model by the way, so you should do everything you can to copy them because they have mastered it. They have really, really nice machines.
You are currently shipping dozens or hundreds of boards per week from a small and stable catalog
You should probably outsource assembly. By ordering in large batches, you will probably be able to get your boards assembled for the same or less than it will cost you to do it yourself. You will easily be able to cover the float of these large inventory batches using the capital you saved by not purchasing an expensive and quickly depreciating machine.
You are just getting started, but you plan on growing quickly and want to make sure you are ready for when the orders start pouring in
You should not buy a pick and place machine. It will be a big expense that you don't need right now. More importantly, it will be a distraction that steals time that you need to be spending on making your products great, selling them, and generally making your customers happy. Get yourself a toaster oven re-flow machine and a good microscope and tweezers and build at least the first 100 boards yourself. Rev after every batch using the inspiration you gained from building the last batch (nothing like placing that resistor 100 times to motivate you to finally design it out). Once your design is stable, you can start ordering small batches from the many great assembly houses until you get to the point where either of the two other scenarios applies.
1 a direct quote from Phillip Torrone of AdaFruit fame talking about their pick-and-place.