Project-Custom Fields

Hi Jhun
Thanks for your questions.
The fields you are looking for are already set up in Project. Try the following and see if it provides the information you are looking for:
In the Gantt Chart view add the cost field (right click on the Task Name column and from the short menu that comes up choose insert column - from the dialog box replace the ID field with the Cost field and click OK)

To make comparisons with budget figures assign your resources to your tasks then just before your project starts save the baseline (tools > tracking > save baseline)

You can now insert the baseline cost, actual cost, and cost variance columns to see what you planned to spend (baseline cost), what you have spent so far (actual cost) what you will spend based on your revised plan (cost) and the difference between what you will spend and your original budget (cost variance)

With regard to your third question the key to Project updating your plan easily is to use links between your tasks and keep fixed dates to a minimum - that way when you change one task Project will reschedule anything impacted by that change for you automatically.

I hope this helps - let us know if you have any further questions.

Kind regards,
Andrew