Sounds like a no brainer to me! Since the starting point is the ZF50, lets go with the M48 (I presume also 0% @ 3years).
Let's also assume the $4K to be available, but the savings account interest is neglegible (for easy math and to not make assumptions).
In 3 years, the $4K ZF will be worth perhaps 50cents on the dollar. So you have a $2K machine or liquidated: $2K [or you can say 3 years of mowing cost $2K ($4K invested minus 2K returned)].
The M48 (assuming 200 hours on it) will be barely broke in and worth likely 75 cents on the $. That means you either have a $4K machine if you come up with the extra $2K or liquidated: you pocket $2K [or count a cost of $2K for 3 years of mowing].
So you have exactly the same financial outcome to have a M48 in your garage (not to mention perhaps the last mower you ever buy); that's still not counting any interest you may have gained on your $4K .
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