That's a tough question. In my case, it's just a bunch of gut-feeling leaps. It's impossible to predict in advance exactly how taxing an app can be to a system

That's a tough question. In my case, it's just a bunch of gut-feeling leaps. It's impossible to predict in advance exactly how taxing an app can be to a system, but you do have some things you can look at analytically: estimated amount of data (start with # of tables and # of rows, per user), how peak-y the traffic will be, what can be cached vs what must be recalculated often, what kind of inbound traffic you can expect to receive based on your marketing strategy. My advice: start small ($100/mo can buy you quite a bit of hardware these days), plan for caching (but keep your code simple to start), structure your systems to be flexible, and always use SSD (saves you a lot of optimization engineering time).