Before going that far, you have to check the PRAM battery is under 4 years old. That can prevent any Mac from booting or properly resetting the PMU.
Also that Mac model was notoriously bad for having flimsy RAM chip latches that would easily get damaged meaning the RAM would not properly seat in the slot.
Lastly, if you were adventurous enough to go and replace the hard drive and/or internal RAM board on the back side of the logicboard, you better have had the special arm adjustment tool that only Authorized Apple Service centers kept in stock, otherwise, you might accidentally damage the cables going into the neck of the machine when you put it back together, making for no display coming out of the machine. That could be remediated, if nothing else got damaged by adding an external VGA display with the mini-VGA adapter. And you would need to be very careful to put back the heat absorbing pads and cooling gel in the right amounts so they are neither acting as an insulator, or as a ventilation problem.