In general you should keep in mind that a quotation is a formal offer and may be deemed to form a contract if accepted. Therefore you should make sure you are clear about what you are providing and the terms under which you are providing it.
In general, anything applicable to the invoice is applicable to a quotation except for things like payments already applied (though it never hurts to put a note in as to how much credit can be applied to the work when invoiced). At a bare minimum, however, you need to have:
A clear description of what is offered, including as appropriate quantity, scope, etc.
A clear description of who is responsible for overruns. This can be further clarified by labelling the document a QUOTATION for fixed bids and an ESTIMATE for estimates of hourly work.
A clear description of payment terms, late fees, etc.
Any other terms of service, either incorporated literally or by reference.
An expiration date, informing the customer how long they have before the rates may need to be rechecked or renegotiated.