While most domains in a niche are in fact taken, there are always domain options available to register or to buy for short money. I have 400+ domains, all registered for some project that I wanted to do at some point.. Id never consider leasing a domain because as you said, once you are done with the lease, ALL of the work you did is now gone and you start over wherever you go. The best case scenario is you end up with a mailing list you can use, worst case scenario you start over.
You mentioned branding, a generic word is a lot harder to brand than a made up word or phrase, look at google.com askjeeves.com, mapquest.com, gofuckyourself.com etc etc. If you need help finding domains let me know if im around I dont mind poking through some available ones for you.
Id rather promote a mediocre domain with a keyword and keep the links, PR, and traffic, when im done.
-Lou
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