If you go so far as to get to the guides you'd be as well replace the chain also. It's not the easiest job however (space limitations included). Are you sure it's the chain as they usually are a hardy enough engine (tritec) it could be valve train / and/or the clutch bearing (has it been serviced etc, and has it the correct grade oil)
You could try putting a slightly heavier grade of oil in it to pressurised the guide tensioner better, and rule the chain in or out. 5W30 is a little thin when they've big miles, you could go up to 10w40
You could also drop the sump and see if there are any plastic /nylon pieces (happens when the guide starts to fail)
To strip it down and replace guides and chain probably would take 6 - 8 hours - easiest to pull the engine and box assembly out - if you can get a mechanic that wants to do it....
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