The central clutch cover loves to break off its clips, so do your best to remove it as carefully and as infrequently as you can. Same with the cover for the ports in the back, if yours still has one. The plastics in general suck, due to age, and like to shatter or break. So, generally, be as careful as is reasonable.
Caps aren't generally a problem with the blackbirds (the codename for the Powerbook 500 series). The rev A logic boards have a few, but to my knowledge, they don't tend to be problematic (yet...) The rev B logic boards replaced two of the three electrolytic capacitors with polymer caps, so that solved the majority of the potential risk. You might need to replace that one cap on the rev B. boards some day, but unlikely it is necessary now.
Regarding the hinges, it is possible to fix them, but it does require some unusual tools. You're better off just replacing them if you can.
NOS batteries for the blackbirds are unlikely to be in any better shape than used batteries at this point. However, they can be re-celled to function like new. You can likely find someone that can provide that service here or elsewhere if you ask around, unless you want to do it yourself. The major issue with these batteries is that they are "intelligent" batteries, which means they have a built-in BMS (Battery Management System) that communicates with the main system. It sounds like a good thing, and these days in modern implementation it is, but for these batteries the primitive BMS can be a potential source of serious issues when the batteries were new, let alone when they are re-celled. However, there are various ways to deal with this. If you want to try and do it yourself, the information is out there, and you can likely find someone here that can help. Although, honestly, this is a task better handled by someone with expertise in the matter. Generally you don't want to store any old batteries in the system that haven't been re-celled since they can and do leak, which can cause a significant amount of damage to the system.
The hard drives, given their age, might still work, but its only a matter of time until they fail. Laptop SCSI drives are somewhat difficult to find and not inexpensive if you do. Plus, they will eventually have the same problems as any mechanical drive and will fail. Some people love them for the authentic experience, personally I fail to see the allure for a blackbird as they are literally the one source of noise for the entire system. There are plenty of options for replacements these days. One is based off of SCSI to IDE drive adapters for mechanical drives which are much larger and newer but, as with all mechanical drives, they will eventually fail (as you might be able to tell, I have something of a bias against mechanical drives, even in vintage systems). These SCSI to IDE adapters can sometimes be used with compact flash memory, but it is hit and miss. There are some modern SSD SCSI drives out there that are compatible with these machines, but they are rare and typically expensive (unless you get lucky). There are a number of SCSI-SD projects in the community over the years that will allow for the use of an SD card of any number of flavors to be used as an internal drive replacement for the blackbirds. At this point they are generally very reasonably in price, stable, and easy to use. They are generally highly recommended as a replacement for a mechanical drive (and it makes the blackbird completely silent.)
You got rather lucky to get a completely intact system, as well as a working power supply together. Given the age of the power supplies, they do tend to fail. When this happens you generally have two options, chop off the end (since it is rather unique) and interface it to a new modern power supply as a substitute, which tends to work well, although some people have run into unexpected issues. The other alternative is to crack open the period-appropriate power supply (in a literal sense, they are ultrasonically welded shut, so there is no way to open them that doesn't involve literally cracking the plastic to open them. Apple apparently intended for people to replace them rather than repair them.) and replace the capacitors in them (and any other components that were damaged when the caps decided to drop dead). It generally isn't too difficult, and if you are handy with a soldering iron you can do it yourself, or it wouldn't be difficult to find someone in the community that can do it for a reasonable cost.
If you want to use Wi-Fi (limited to 802.11b and WEP or no encryption). you need what's known as a Rev. C card cage, which goes in the left hand (if you are facing the system) battery bay. The Rev C. card cage is the only one that can handle Wi-Fi. The Rev A & B card cages can't. The Rev. C card cages tend to be expensive and somewhat difficult to find. The card cages in general give you access to most Type I, II, & III PCMCIA cards made for Macs (don't confuse with Cardbus, this is strictly PCMCIA. Cardbus cards are not compatible with the blackbird PCMCIA card cage), with the Rev. C card cage giving the largest range of compatible cards.
The CPU exists on a daughtercard, which allows you to upgrade the CPU from a 68LC040 to a full 68040 (although this type of daughtercard is rare and typically expensive), or a PowerPC 603e CPU ranging in speed from 100 to 183 MHz. If you want to stick to Apple parts only, the only PowerPC upgrade available is the 100 MHz upgrade. The rest of the higher speed upgrades are from third party hardware companies. The most common upgrade is a third party PowerPC 603e 167 Mhz daughtercard.
If you really want to get deep into "optimizing" your system there are two different types of LCD's for the 540c, the Toshiba LCDs tend to have a lower dot pitch so the screen is seen as "sharper", which the Sharp LCD is generally seen as having better color reproduction. Swapping the LCD would allow you to choose which one meets your needs.
Generally they are fairly robust systems and are a joy to use. The blackbirds, especially the 540c, are arguably (since everyone has an opinion on this) the most versatile and useful of the 68k system ever released by Apple.
That's all that comes to mind at the moment.