depreciation policy

My client has a diversified business. More than half of their Balance Sheet is made up fixed assets (PPE). They deploy the bigger chuck of this PPE hiring it out as one of their lines of business.Furthermore,some machinery and equipment though still working have completed the manufacturers' lifespan. Surely, working for more than 30 years is outside most conventional depreciation lifespans.

Can someone out there please advise what depreciation policy I can recommend to them. Should it be based on useful lifespan or quantities produced? Both are applicable, I think, but I am concerned with using more than one base of depreciation as it is tedious.