I want to set up something like a hosted CMS where customers can create normal sites or e-commerce sites (they choose what they want). I have been advice that Drupal is the option for me(Other options are welcomed). I am in the process of making some decisions so I want to have a general idea before I make my final decision.
My questions are:
Am I going to be able to add ANY features/functionalities I want to Drupal to satisfy the needs of my customers. If I say ANY, I mean ANY functionality I can think of. Or will Drupal limits me of the features I can add to it?
Concerning the database, does Drupal allows additional tables and columns to be added in order to implement certain functionalities or everything is fixed?
Am novice in the technical aspects of Drupal, and CMS in general.
First, Drupal is CMS based on php so you can do everything but either you'll not follow Drupal standards to write a native php inside it or you'll use Drupal api and do custom modules so it'll take lots of time.
About your questions:
you are able to do anything you want without limits "create table fields, write native php" but that all are not Drupal standards. You have to decide time or professional work for sure according to your client.
As I said at the previous step you can create tables either by going to phpMyAdmin and create it manual or to do custom module to create your table and go your functionality in a centralized place.