315 hours!?!?!?!?!

That thing is a baby. Hardly broken in. Mine is a 1990 and has 1420 hours, which is pretty low for it's age (or so I thought until you showed up). :winky:
I would not power wash anything on or under the hood. Too much potential for collateral damage. Just use a regular garden hose with a spray nozzle and that should be plenty of pressure. The grass screen can be removed entirely for cleaning. Then wash out the radiator from the fan side to blow out any debris lodged in the fins. Make sure that oil cooler is clean too.
With respect to the lift arm strut upper link, you got lucky. Since it is the adjustable side, you should be able to just replace the upper end, rather than the whole strut. Check with your local TSC or implement dealer. Is it a JD part? If so, you should be able to get it through the local JD dealer. Be sure to lube all the ball joints on the TPL arms and the top link.