Security considerations - office website/portal on GAE
There is always the choice between usabiity and secutity. The more security features you implent, the more difficult it gets to use it.

can we host the apps on GAE (appspot.com) with https?

Yes, but not on your own domain, only on appspot.com. If you are serving your app off of an own domain, you must direct all secure traffic through your app's appspot domain (on your own domain, you'd have to buy a SSL certificate, and you would need a dedicated IP etc.). If you really have to, there are ways to route SSL traffic over your own domain, but as this requires another server running something like stunnel, it gives attackers another attack target.
If your app has username/password authentication, the app is really as safe as how safely one keeps his password, if you have no bugs in your code that could be exploited. About the "hackish way": on GAE, you don't have to care about server security, the only possible attack target is your code.
These are some strategies for securing your app:

good QA and code review to find critical bugs; Django has already built-in protection against most trivial attacks like XSRF and SQL injection, so look at the parts of your own code that are related to critical data and authentication
think of other authentication methods like client side certificates (easy to use for the end user, most browser support this natively and modern operating systems have a certificate storage; probably not an easy thing to do on GAE)
the weakest point of every secure enviromnent is the user, so you should inform the users about good practices on handling sensitive data and passwords (BTW, requiring a password change every few months does not improves security at all as it usally results in users writing down their passwords as they can't remember it, you loose more security than you gain)
you should have good intrusion detection to lock out an attacker as soon as possible, as example behaviour analysis; Example: if a user from the USA logs in from an IP in Estonia, this is suspicious
network access restrictions: you could block all IP ranges except those from your enterprise of accessing critical data, if a password gets leaked, this minimizes the possible impact
improve end user security: if one of the users have a trojan on their computer that makes screen captures or keylogs, all your security is lost as the attacker could just watch the user while he's vieweing sensitive data; you should have a good security police in your enterprise
force users to access your site over SSL, you should not let the users choose if they prefer security ocer comfort of not
If one needs to create an office website (that serves as a platform for clients/customers/employees) to login and access shared data, what are the security considerations.
to give you some more detail, 
The office portal has been developed in django/python and hosted through GAE. Essentially, the end point comes with a login/password to enter into the portal and access data.
I would like to know:
a) what are the things we can do to bring in a high level of security. Essentially the data is critical and hence need to be accessed by authorized people only. So would like to make it such that "The app is as safe as - how safely one keeps his password. Meaning, the only way to enter the system (unauthorized) is through a password leak (by the person) and not in any hackish way."  :)
b) can we host the apps on GAE (appspot.com) with https?
c) are there better ways to secure other than passwords (i have heard about ssh keys/certificates). But the ultimate users may not be highly tech savvy.