Consulting about implementing ad-classified engine from the ground?

Hi guys,

I’d like to implement a professional advertising classified engine in
Ruby
on Rails with cutting-edge technologies from the ground similar to
Osclass
[http://osclass.org] but scalability as Craigslist, flexibility and
customizability (integratable, theme customizable & plug-inable) as
Spree.

I want to know your opinions about full of the project infrastructure.
It
includes about selecting best technologies for backend, middleware and
frontend sides, best gems (authentication/authorization, theme
customizable
& plug-inable solutions, TDD, …) and so on.

Can anyone gives me your best advices? And please provide with resources
for best-practice.

Thanks!

P.S. Maybe It will be an open-source project under MIT license.