You should beware that, there is no need to overwhelm the system by providing unnecessary security. You might think that there is no such a thing like unnecessary security, but based  on what you intend to do, and what type of information you pass over the net, i.e. overloading system by unrequited cryptography will only impact the performance and response  time of your system, there are other examples, but just in a nut shell, choose your requirement and define your security  needs based on those and avoid unnecessary overloaded security computations.

This is up to you, look at amazon.com, they also accept bank drafts or cheque, but they will only deliver the produce on validation of the cheque. Basically you own your own policy

To my knowledge, the best is to prototype something, and perform on multiple platform  and provide a performance analysis, also review the characteristics of the chosen platforms and provide a comparison report, that would very helpful when making a decision + have a look at future both in your needs and technology  wise as how in future the chosen platform will let you to migrate  to new technology.

I guess again refer to my answer 3, but again don’t try to built in the security in the DB as this is not a portable solution so if you want to (for cost reasons) choose a DB at this point of time, built-in security might not be portable and you might end up with rewriting security layers again which will not be a wise decision.

I guess this is your view side of the application so if you are careful enough to avoid putting any sort of logic into the view, then that would be easier to produce jsp  or asp, all that matters is to have an adapter to produce a new interface (view) if required. Think more like an MVC sort of approach

You can always sign your code (either binary or text), but this is your source  code, if this is not an open source application, why do you worry? If not only go through legal approaches to include the necessary headers.

Once you have chosen the platform + DB and functionality requirements, you can calculate the cost of implementation, housekeeping, support and additional services (to the your application and your customers)