Aged domains and aged sites are 2 very different things in the eyes of the SEs these days for the msot part, which is most likely why you aren't getting any extra as far as more $$$ to more years the domain has been reg'd.
As far as making sure links stay up, really all you can do is deal with people you trust - even a contract if you are dealing with people outside your own country is hard to uphold (and it's a pita upholding one even in your own country/town).
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