Dell Latitude D820

Hi! This is my first post here.

I have found this D820 in a dumpster (literally it was sitting ringht on top of the bin so I took it, was just a dumb find as I was taking out the garbage, I do not dumpseter-dive) and it has some obvious problems. The LCD was cracked and the keyboard was messed up (had missing keys and was either very dirty or had some waterdamage). Either way, I plugged an external monitor to it and plugged another Dell power adapter (a 90W model from my XPS M1530) I had lying around since the battery is either dead or in a deep discharge state. I tried powering the machine up, but besides the power LED turning on for a couple of seconds, nothing happened. Could it be the motherboard or shall I first try replacing the LCD and keyboard? Please don't go "its not worth it" because I can always buy SH parts.

I am quite tech-savvy, I already took it apart to check it for further damage on the motherboard and other components. Nothing appears to be wrong, the labels on some of the parts do not appear to have sustained any heat-damage. The whole machine only appears to be a bit dusty, but I dealt with that. It has all the original labels on it  so it wouldn't be a problem to type in the service tag, if needed.