In the Power Store settings, you can set the Gateway to none so it will not process the transaction and act as a quote system.
on the webassist/plugins/shopping_cart/checkout_form.php page, the checkout button is a standard submit form button:
<input type="submit" class="ps_buttons" name="Checkout" id="Checkout" value="Checkout" alt="Checkout" onClick="formSubmit()" />
you can change the text of the button by editing the value:
<input type="submit" class="ps_buttons" name="Checkout" id="Checkout" value="submit for quote" alt="Checkout" onClick="formSubmit()" />