Huh, I hadn't realised that there was a Pentium MMX version - I'd always thought it was just the Stylistic LT P600 (P3-600) and C500 (Celeron 500).

It should be pretty easy to replace the hard disk drive, on my LT P600 there's a cover that opens with two screws on the back. The HD cover slides off and behind it there's the hard drive connected up with a flex cable. That might be delicate and it's probably why that isn't listed as a field replaceable part:

But that simply pulls up to disconnect and inside is a 9.5mm height 2.5" IDE hard drive. Anything like a compact flash > 44pin IDE adapter would fit in there nicely - which I need to do to this one really soon because I put one of those Toshiba 30xxGAS hard drives in there a while ago, which now won't spin up

The chassis design looks the same between the two so hopefully it's easy to for you to swap out also 😀

The BIOS battery is a bit of a task to get to - I had to take off the mainboard to get at it, but the cell on mine doesn't need replacing - it looks bigger than a regular CR2032 lithium cell:

Cutting away the heatshrink, it's a BR2330 in mine. If it's a "BR" or "VL" cell then it's rechargeable and only needs replacing if it's not charging anymore. They don't tend to leak badly and it only affects the real time clock if it's not working.

While taking it apart though, I found something to sort out, it's a Varta!

Because this little tablet (and your original LT) are meant for 'fleet' use out in the field, the batteries are easily swappable so it's got this little 7.2v varta cell to keep the RAM running while it's in standby. Allowing the battery to be swapped out without a charger nearby. These do leak and do damage and yours should have similar, which should be removed right away. I had to clean up the plastic casing which had some blue residue on it from the battery.
The battery is remote from the mainboard and cable is really long though so the damage is minimal and easily cleaned up.

When disassembling, beware of the relatively fragile plastic - it's a Polycarbonate + Glass Fibre mix which was quite strong and resilient when new but goes brittle over time. Just slide it carefully over the PCMCIA / CF card side (may need to press the CF eject button if it's got one) and move it carefully over the audio jacks. On mine, there are two long black screws at the diagonal corners, three very narrow black screws near the dock connector, then three short black screws in the battery bay. Inside should be silver screws that hold the mainboard in. The ones with the washers are for the PCMCIA slot.

That Stylistic LT is cool, it should have a DOS compatible soundcard in it with that ES1879 - afaik that's an ISA sound chip. That's something I wish I could use this LT P600 for.

Thermalwrong - Thank you for the response, that was very helpful and more detailed than I was expecting! I'll contact the seller to see if it has the same hard drive cover as yours. There isn't a lot of information on these tablets online, but it seems like they are well liked by whoever has them. Seems like this would be worth picking up!