not generally an issue, except for getting higher wattage supplies. Some of the Older models had somewhat special shape power supplies.
Of course space for cooling can become an issue.
Quite frankly if you really expect to grow, get a mid ATX case and move everything into that.
I just bought an HP desktop at a real good price and may have to install a couple of peripherals. Floppy drives don't come standard anymore, but I still use one, and I'm used to two optical drives and the new machine only has one. If I decide I can't live without a floppy drive and another CD or DVD drive, I may need a bigger power supply, too.
At any rate, I've heard in the past that HP computers have proprietary parts issues that keep an owner from installing aftermarket parts. Is this the case?