1. when i set up the new, will my previous setting take hold, or do i have to re-set everything. including a password, etc.
The new Extreme will not automatically configure itself with the old Extreme's configuration settings. One option is to export the old settings, and then, import them into the new Extreme. This can be done using the AirPort Utility. You will find the import & export options on the utility's menu bar on the File menu.
2. the extreme that i currently have (gen 5) can that now be used as an extender?
Yes, you can use the 5G Extreme as an extender of the new 802.11ac Extreme's Wi-Fi network. Note you will NOT be able to extend the 802.11ac radio in this manner.
3. with the size/layout of my house (and my business where i will set up a new one as well) my plan is to use the new airport base station and the old one as an extender, along with a new airport express as well. my thoughts are that this should give me enough range through the house with this set up. Or is it overkill?
The number of base stations required will depend on both the exact makeup of your house and where you want to have Wi-Fi access. Keep in mind that an extended network works somewhat like a wheel, with the base station to be extended at the hub of that wheel and each of the extending base stations along one of the wheel's spokes. In this configuration, you cannot configure the base stations in a daisy-chain method in order to extend the network futher along any of the spokes.
Unfortunately, just exporting and importing, does not "capture" all of the settings ... Especially if the AirPort models are different. Yes, you may need to re-setup the Guest Network to get back to your original configuration.