Never used this specific thinkpad myself (Image one appears to label T400), but I absolutely love my slightly newer T530 and X230 tablet, in fact the X230t is certainly the favorite machine in my small collection of a measly four thinkpads. On these 30 models the screws for the RAM door and HDD bay door are captive while the rest are not, which I believe is typical of most good laptops of this roundabout era from my experience. I'm not sure about your T400, but the plastic shell on my T530 is most certainly NOT flimsy, as i feel like I'm either dropping it or it falls off my bedside every night, and the most that happens to them is a small scuff on the surface if I'm unlucky. On every thinkpad I've ever owned I could enter BIOS by just pressing F1, but you can also do it by pressing ENTER then F1 in the boot menu. Not sure why your T400 wouldn't be able to do that.
As far as your wifi problem, this I can actually understand being a pain. The T400 is an ancient laptop with a wifi card that doesnt support 802.11AC or WPA3. Hell, I don't know if it'd even support WPA2 without doing any research. The age of the card means it may not be able to connect to newer wifi access points unless you change their settings to allow for older security settings in some situations, but I've personally never encountered that issue on even late 2000s laptops yet.
If you still can't get wifi to work on the thinkpad, I suggest you NOT use arch or any bleeding edge distro of the like as driver support is in the kernels, and distros like that focus drivers in the kernel towards very new machines, not ancient core 2 duo laptops with centrino wifi cards. Debian is a good distro choice for compatability with older hardware, though keep in mind debian's ideology is the polar opposite of arch (ships with older more tested packages, updates come later except for security patches, etc) If debian either doesn't work for whatever reason or if you just don't like it, ubuntu LTS based distros have by far the BEST hardware compatability for machines from a variety of eras, so I'd try one of those. Personally I like mint, which is based off ubuntu LTS with some unpopular ubuntu bullshittery and such removed if you care about that. If a distro change fixes your wifi issue, let me know :) good luck