Where to start after dropping my MBP, which no does not boot anymore

I bought a non working 2009 17"MBP with aim to fix it. The previous owner said:

"I dropped the computer in a laptop bag off my shoulder. It only turned on completely and functioned normally one time afterwards and only after playing with it and holding down the power button for a long time after taking the battery out, but it worked completely fine. I shut it down and after that I've only been able to turn it on for a second or two before it shuts off." (end quote)

The green charging light on the mag is on, so SMC is communicating.

I am ready to "go in" , I have my brand new rework station , flux, and even schematics and board view, but would appreciate pointers as to where to start in finding what is probably a broken solder joint....besides checking power rail voltages and looking for obvious physical damage....

I will try to put in a known good HDD. However, as far as I know a Mac will boot even without any HDD, at least to the point where it shows the folder /question mark.?