"the d o c was the same except for parts when required"
That's the problem. I suspect the 'parts' you'll require (or overhauls etc) will easily add up to more than the depreciation you would suffer on the R44, given your 3 year/600 hour sum.
Put it this way: I've recently sold a 37 year old JetRanger for c.£35k more than I paid for it in 2002, with around 500 hours more TT. But I'd hate to do the sum of how much I spent in between times.....but it sure would have been a better deal to buy an R44!