Hi there,
First and foremost, thanks for posting the job.
Now moving towards your project requirements, where you want to get the best suggestion for the website you h ave in magento older version, I would like to put some points which might clear things about the website:
1) Upgrading: Before we talk about the up gradation i will say that, whenever we update the website to new version all the modules which are custom or has been customized will not work in the upgraded version, thus we need to code them from scratch keeping the new version compatibility in mind, and this will consume so much time.
2) Creating it from scratch in latest version: This option sounds fair and also it would be beneficial as the one who is working on this project will start everything from scratch and the benefits he can take from the older version is the data, one can easily take the data from the older site that will include products, users etc, so its one of the advantage of creating it from scratch.
3) Custom framework like Zend: This also sounds fair, but here the costing and the time will raise as everything has to be done from scratch while if we use the magento then half of the efforts could be saved as well as cost, the only advantage using a zend is the customizable nature of this framework, if you have a lot of customization we would strongly prefer using the zend.
Keeping all these points i think you have got a fair idea and thus we would like to discuss on Skype.
Regards