Upgrading my PC

You obviously had a cheap PSU then. You wanna go for quality, not for quantity. An consumer grade HDD usually consumes about 5-7 W, and if a PSU blows up or causes the entire system not to work anymore, just because you're adding another HDD, then it's pretty clear that the PSU either was defective or of poor quality. Beside that, fans even consume less power (2-4 W, depending on size and RPM).

Given the fact that the OP doesn't want to overclock, nor to use SLI/CF and is currently using an HD 7850, his system will consume roughly about 200 W when under 100% load (without peripherals). Now let's get bonkers and add 10 HDDs (each with 5 W), a whopping 10 fans (lulz; each with 4 W), a fan controller (which will be needed when trying to use more than 8+ fans nowadays; with another 5 W) and we'll end up at 300 W. Even if we add another 6 USB devices with a power consumption of 10 W we're only at 360 W.

Bottom line: If the OP decides to upgrade his GPU in a year or two, and if that new GPU consumes 250 W (that's GTX 780 Ti/GTX Titan level; HD 7850 consumes ~ 130 W), he will never get over 500 W of total power consumption. However he's probably only going to use 1 SSD + 2-4 HDDs and 2-3 fans max, so a 500-550 W PSU will work just fine and will always have enough capacities for CPU, GPU, HDD and fan upgrades.