You need to watch a bunch of barn find (and barn find road trip) videos, its actually really common.

Change all fluids (or minimum the oil), drain gas and fill with some new gas, throw in your favorite snake oil of choice to feel better (mmo, atf, seafoam, injector cleaner, whatever), spray a bunch of starter fluid in and crank like crazy.

Once its up and running let it idle (hopefully you get this far) and get it to full operating temperature. You want to inspect it for leaks especially coolant hoses.

I would not trust driving it until brakes are completely done - at least replace fluid, check all rubber lines, compress pistons to make sure they are not stuck etc etc.

Sometimes it is easier to just sell it and buy something else, depending on condition and storage every rubber seal hose component etc might fail one by one.

Windows ME? Single threaded OS. That one core is probably only giving you a pathetic 36kfps in games. Need to upgrade to an i9 at least. /s

Also nice work. I dont know why you want to do this but nice work.

CPU temps? Could be throttling from cooler issues - sticker, not seated correctly etc or case does not have enough flow.

SSD/HDD? If your running Windows or your games off a HDD that would be your issues.

Whatever gives you the most for your money and fits your requirements.

I would lean to AMD just for the upgrade path if both options are around the same price/performance.