Yeah, charging 50$ to enable hardware they already sold to you is quite something. My model didn't have that option, it only has a bluetooth radio.

Can't complain about the build quality though. It's been rock solid for more than 2 years now, running on 4 rechargeable batteries which seem to last quite long. My first device was from a batch that apparently had radio problems that didn't affect me, but they replaced it proactively anyway. Can't really say anything bad about them from personal experience...

I have a previous version Nuki with a keypad. I think it works great, also locks my door automatically at night. I don't have it connected to anything else however, to limit attack surfaces. You can still use the app, as it uses bluetooth. That in-app purchase is pretty shitty, though the lock itself is not that expensive compared to competitors. I would advise to search for discount codes or wait for promotions, which happen frequently.

Do remember that it requires a key into the cilinder on the inside, so if you don't have a double cylinder you can't use your key on the outside. Which means you can lock yourself out when the batteries die. You should get a cylinder with an emergency function, so you can use a key on one side when a key is present on the other side.

I used to buy external Seagate HDD's and pry open the enclosure. Inside it's just a regular 2,5" HDD with a small sata>usb board. Last time I checked these are still available and might be easier to find (and cheaper). But yeah, you don't have many models to choose from these days. You could use SSD's which will save a bit of power but they are vastly more expensive. I don't think the money you'll save on power will offset that.

I built a system ground up with a focus on power-usage a few years ago. You can go far down the rabbit hole when you start googling, but the key take-aways for me are:

• Eliminate features you don't really need. E.g. do you realy need a RAID 10 configuration with 4 disks? Or can you get by with 2, or maybe even 1 (which might mean you experience downtime while waiting for a new HDD, but you do keep backups right?).
• Standard 300-400w ATX power supplies are inefficient under low loads (meaning much AC goes to waste converting it to DC). Use a PicoPSU (or a motherboard that runs on DC). Search for a high-quality power adapter (e.g. Leicke) that's rated for low loads. Just make sure your PSU can handle booting, when your system will draw much more.
• Install drivers for motherboard features you don't need (like soundcards or wifi) and then switch them off in the OS as this usually sets them in a more power-efficient state rather than disabling them in BIOS.
• SSD's use less power than HDD's but get expensive for higher capacities. I use a Nvme SSD as a startup drive and to host my containers, and 2,5" 5400rpm HDD's for storage. Those disks are slow but get the job done, and despite running 24/7 there hasn't been any failures.
• I installed a passive cooling-block on the CPU and optimized the airflow in the case. It has 3 (small, as it's a 1u server case) case fans that are controlled by fancontrol and only kick in when the cpu's temp is > 60°C, which is not very oten. This also means it's very quiet ;-).
• Make sure to configure all power settings in the OS of your choice (powertop and tlp for linux).

Anecdotal, but mine (a 4TB one) is running fine (knocks on wood) for 5+ years now. It's in my server and doesn't spin down. Low power usage as well. Very happy with it...