In terms of opening, it depends on the case. There are some very easy no-tools PC cases out there. All Dell servers, for example, are just a lever to open (I mention them since we buy a lot).
However that aside easy of upgradeability isn't about how easy you can get the side off, I mean really if a thumb screw vexes you, you are being silly. It is about component availability and this has always been a massive Mac problem. Things like custom powersupplies, custom video card BIOSes, that sort of thing, and of course fuck-all available from Apple. When you get a PC, particularly a high end one, you've got all kinds of options. With a good manufacturer, they will sell you the stuff, as well as your ability to get it aftermarket. Like with a Dell workstation Dell will sell you, after the fact, addon processors, memory, GPUs, HDDs, SSDs, RAID controllers, HBAs, network adapters, power supplies, and so on for your system. All of them come with full warranty support though Dell, of course.
They don't have what you want, or don't have it for a good price? No problem, you can get it all aftermarket. Nothing special needed, buy the regular stuff from any vendor.
You can really upgrade the hell out of a PC, and keep doing it, if you want. I haven't bought a new desktop in like 8 years, yet it is still very much top of the line. What happens is I just replace components as needed. I get a new GPU every 18ish months, new HDDs if I run out of space or if something is faster enough to catch my interest (like my SSD), a new audio card when I see one with features I want, a new motherboard/CPU every 2ish years, new RAM if the motherboard needs it, new PSU should power requirements change (hasn't happened) and a new case never because I like mine. So even when the core, the CPU and motherboard, get upgraded it isn't a new system. I can keep the case, PSU, GPU, sound card, drives, and all that jazz.
Now I'm not saying this is how people should do it, but that is a demonstration of what real upgradeability means. It is the ability to upgrade any component when a new one comes out more or less, and to do so with anything as much as needed. Not the ability to take the case off and put in more RAM.
In terms of network storage I suppose... But what? OS-X can't act as an iSCSI initiator so you can't use any of the nice high end iSCSI arrays (like an Equallogic) or something. No 10gig so no FCoE. Apple doesn't make storage arrays and nothing else seems to support AFP. So... You buy a Windows server and use CIFS? Last I tried, CIFS performance wasn't great on the Mac, but whatever.
Macs really aren't that well designed for network storage on account of not having anything out there for them. I mean generally for network storage you either want a NAS that speaks the protocol your OS likes, and for OS-X that's AFP which is not popular, or for higher performance/lower latency you hook up using iSCSI or FCoE. iSCSI is real popular because gig (and bonded gig) are options and you can run it over your regular network, even over the Internet if necessary. Most OSes (Windows, Linux, BSD, Solaris, VMWare) can act as initiators and talk to an iSCSI target (most of them can be targets too if you want), but not OS-X, it has no iSCSI support.
I mean they'll talk to a CIFS share if you are just looking for a place to put stuff, but given the lack of space I presume you are talking about networked storage in a high performance capacity, using it online like local storage. That really only works well with high performance stuff and that they do not seem to have.