I'm going to self-appoint myself as somewhat as an expert on the 2400 since I have been through so many of them.
First, some unabashed adoration: these are probably my favorite of the vintage PowerBooks, certainly my favorite of the pre-WallStreet PPC models, because they have an excellent combination of speed, style, portability, expandability, upgradability, and arguably the best keyboard of any PowerBook (in contention with the 1400 and WS/PDQ, though admittedly the 2400's can be frustrating to people whose stats exceed about 5'6" tall or about 160lbs since the keys are so tiny and closely-spaced, but its firmness, short travel, crisp response, and unique inverted-T arrow keys cannot be ignored). These are generally pretty solid computers compared to their contemporaries: their cases are usually in one piece, the I/O door is usually attached, and they don't normally suffer from any of the hinge-related failures of the rest of the PowerBooks built before or since. The worst thing about them is that the floppy drive usually isn't included anymore, and since it's a model-specific unit with a special cable it's kind of a pain to replace. They also came with a special small 2400-specific power adapter that may have been lost and replaced at some point with a yo-yo or one of the big bricks from the 1400 or 3400. You can readily use one of the replacement adapters, of course, but the matching smallness of the original adapter helps make the set.
The big failure points on these (and other models with the pea pod-esque NiMH PRAM battery such as the 3400) are undoubtedly the PRAM batteries, which leak and destroy the logic board, and also LCD panel degradation, primarily suspected to be the result of poor environmental storage (too hot and humid, possibly stored alongside certain chemicals or with no ventilation). These problems are not unique to the 2400 since I have stacks of computers and other devices with leaked batteries and bubbled-up LCD panels. With preventative maintenance and proper storage you can likely avoid these problems, though.
As with most PowerBooks, capacitors are generally solid except for a handful of tanks in the power supply section. They usually don't cause problems, especially here on the 2400's charge card since most of them are excellent Sanyo OSCON types rather than the standard electrolytics usually populating most computers. IBM Japan did an excellent job engineering these and sourcing quality components.
However, small problems you're likely to experience will be total degradation of the rubber bumpers on both the main unit and the floppy drive, the floppy drive door missing or with busted hinges (I'd say 90% of these are broken by now), missing screw covers on the clutch cover bar, trackpad button unresponsive or otherwise uncooperative, and occasionally broken I/O doors or display latches.
Full disassembly is required to remove the PRAM battery. I never bother to replace them, simply remove them, because a. they're an additional expense; b. the new ones are likely going to go bad in another decade or so necessitating further surgery on equipment that may be too brittle to endure it; and c. I never use them often enough for the battery to be effective: either the computer is plugged in/has a charged main battery while I use it, or it's in storage where the battery will be depleted in a week or so anyway. If I was a road warrior in 1997 and was sleep-swapping the main batteries on the go, yeah, a good PRAM battery would be essential. But they're hobbyist playthings at this point so it's not important. You have to use weird workarounds for the proper day and time to be displayed in classic Mac OS anymore anyway.
While you're in there you can replace the hard drive as mentioned with something a little more spacious and faster. You may notice that there's a raised area with a thermal pad on the shielding beneath the hard drive. Obviously a modern 9.5mm drive won't contact this so the drive may run a smidge warmer than the original 12.5mm drive would have, since the original drive would have been making contact with the thermal pad to aid in heat dissipation. Anyway if you plan to use SCSI Disk Mode, ensure you partition the first 4GB of the drive and only use that when connected to a different Mac or you'll get massive data corruption on anything past the 4GB partition (the host Mac will mount all partitions, so just eject everything except the first 4GB partition and you'll be fine). This is true of any pre-G3 IDE PowerBook owing to a ROM bug; there's no getting around it.
You can hack it to support CardBus, depending on your specific logic board revision, by either removing the two yellow wires above the RAM slot to the right of the ROM chips, or unsoldering C375 and C377 in the same general area.
You can also install a G3 upgrade if you can find one that's reasonably priced. The 320MHz and especially the 400MHz ones are rarest and most expensive and were primarily sold in Japan, but the 240MHz ones show up on eBay every now and again.
If you want to upgrade RAM, the official maximum is 80MB using a 64MB EDO SODIMM, but there exists a custom 96MB SODIMM that's about as common as the G3 upgrades, and that will take you up to 112MB. This combination would allow you to boot OS X via XPostFacto if you're into it. However, DO NOT try to use a normal SDRAM SODIMM or you may burn something up since voltages are different.
Speaking of burning something up, the logic board and power card on those things are littered with fuses. If you have a problem with something (no boot, no hard drive, no battery charge, etc), check your fuses. Usually the bad ones will have a black dot in the middle but you'll want to check with a multimeter anyway.
The trackpad clicker is always a little loose-feeling. Originally there were some foam pads underneath it to reduce this but they've turned to powder after 20-some years. If it's not working the button assembly itself can be cleaned but it's a touchy job. To clean it, you'll have to disassemble the trackpad, unstick the ribbon cable assembly, carefully peel back the clear tape over the button contacts, remove the flexible metal button from the tape, and clean both the underside of the flexible button and the contact on the ribbon cable base without damaging the tape. Then you'll have to reposition the button atop the base contact and reapply the tape without skewing the button or pressing too hard on it. Alternatively you can replace the button with a different physical button but this involves some soldering and careful selection of a new button/modification of the clicker post to get it to fit properly. To prevent damage to the clicker button once you've repaired it, replace the bumpers on the LCD panel and/or don't close the lid; excessive pressure on the button while the lid is closed is usually what damages the button in the first place.
The LCD panel is an IBM 11.4" CTFT unit unique to the 2400c (at least among other Macs), so try not to hurt it because replacements are not easy to source.