Its not worth the energy cost, honestly. Plus you'll be quite limited on memory, which reduces the potential uses. Any $200 minipc from the last 5 years would be a better buy.
They're pretty slow, come with less RAM and replacement parts have become more expensive, because they're not as available as ones for modern computers. Also my N54l consumes like 45W in idle...
I wouldn't spend money on one, today. You can probably get something way faster and more capable for less money. Security isn't an issue though. That mostly depends on the operating system you put on it, not the hardware. The security issues in the processor and such, should have been fixed by microcode and software updates.
So I still have one from back in the days. But I wouldn't run it 24/7. I've built a small server like 8 years ago that consumes a third of the power and is far better in every regard. I use the Microserver to store backups and just turn it on every few weeks. But interacting with it feels sluggish and copying files over USB 2.0 takes hours and hours. It doesn't feel like it can make use of the Gigabit ethernet, and it took me some effort to install because you need to do additional things, patch the BIOS to unlock full SATA speed.
Get one if it's free and electricity is cheap whereever you are. But don't spend too much money on an really old computer. The sweetspot of buying used computers is somewhere around 5 years, not 15.
Other than that: An Intel NUC, one of those cheap chinese Mini-PCs from Amazon where you get 16GB of RAM, a fairly recent processor generation and 512GB SSD for like $250 or my advice: get a refurbished laptop for $250. That's energy efficient by design and has everything on board. And available in abundance.
Downsides of these approaches: You don't get a lot of SATA ports for harddisks, if at all... So for storage, I wouldn't consider those. So it's gonna be an old PC, Server or NAS. Comparing mainboards and energy efficiency isn't easy. That's why I rely on PC magazines. But that's for new stuff... Not used components. So tipps from the internet are probably your best bet.
If you're not from a country where electricity is that expensive, you might want to have a look at some of those refurbished PC shops. An server or a Dell Workstation from 5 years ago should be affordable.
I've had N40L and N54L, still running one, and I would absolutely not suggest buying one. They're too old and underpowered, and they're honestly quite inconvenient to service. If you get one for free, then sure, but if you're going to spend money, you can get something cheap, more powerful, and easier to work on, like a used optiplex.
I was just thinking yesterday that I should replace my one remaining N40L instead of waiting for it to die.
I got one 1-2 years ago for ~250€, with a Xeon and 4GB of RAM.
I use it as a "NAS on steroid": basically a NAS-like storage + data processing server (Paperless-NGX, downloader (games I bought on DRM free stores + Usenet), Syncthing middleman...)
It's exactly what I wanted (with the limitation I had) so it's perfect for me (it's still on the 4GB of RAM and I don't need more even with the dozen of container I run).
If the form factor is irrelevant for you, just get the equivalent tower (normal HP Proliant Gen8, or something more recent) for half the price with the same spec, you're paying basically double for the MicroServer form factor.
From a security standpoint, it's irrelevant. Yes the CPU are vulnerable to all the shitty Intel flaws (that Intel thought would never be discovered), but they're all solved via kernel mitigation (that cost you 50% of the performance the CPU originally had)
Be warry of 3 things if you buy it:
Finding the cable to use an SSD in the ODD slot is harder than you think (only a single brand make it), it's also harder than you think to configure the raid card to use it (and it sometimes but rarely resets) - this is only relevant if you want to use that slot
First 2 sata slots are SATA3, others 2 are SATA2 (and they're not fast), ODD slot is SATA2, I think, I might be wrong
The motherboard chipset has a bug: you must disable one of the virtalization option (I don't remember which) if you virtualize / run a RDP docker container, otherwise the system will freeze after 30-1h of running said virtualized stuff in the background (took me days to find the solution, it's not documented anywhere on the internet except for like one place)
I wouldn't get one unless it was dirt cheap. The CPU wasn't that great when it was new, now an ARM SBC will outperform it using a fraction of the power.
It can be fine, I'm using a comparable machine, you have to do the math for whether the power bills are worth it. What cpu does it have and how hard do you plan to run it?
I know everyone has their own opinions of them but I'm a fan for what they are. Right now I have 3 of them that I've gathered over the years (one with ESXi hosting my firewall, one with TrueNas for backups, and one with ProxMox for a few LXCs).
Overall, they are great little boxes, I had three of them in my living room for years when I was renting and they were pretty much completely silent after boot. The dual core celeron that comes with it works, but can be upgraded to a Xeon e3-1265l v2 (quad core + HT) for $25-50. RAM I think maxes at 16GB, but if you want a box to run a dozen light services or so, its not a bad box (insanely quiet and pretty power efficient).
I have a big boi ProLiant DL380e Gen8 and it's pretty good as long as you have a dedicated room for it as it's really loud. If you get them for really cheap like I did (I got mine maxed out for less than 100$), then yeah it's a good deal.
It's not amazing on power consumption either with like 130W at idle... I'd imagine the smaller ones using use less power, but I wouldn't count on it, as it is still old hardware. However, if the price is right, then it could absolutely be worth it... If you can withstand the noise.
And looking at the specs of the N36L, for the same price you could very easily scrounge together a custom build from 2nd hand parts, which will be quieter, faster and most likely use less power...