How to setup a guest wifi using 2 x DD-WRT routers?

Ok this is for a small business. We have one router running dd-wrt attached to that one are about 10 computers (wired). Then we also have office wifi to which one mobile tablet is connected to.

Now the goal is to add a secure separate guest wifi. I have another older router running dd-wrt which would be great for that. The dsl modem is connected to the office main router, then a cable would connect the guest wifi router to that main router.
This is for a doctor's office so the guest wifi must not be able to tap into the office wifi/network but just use the internet connection (ideally with priority given to the office networks).

Can anyone guide me through the settings what to enable, how to connect and setup those 2 routers. Some question I have specifically
1) Do I enable DHCP on the guest router as well, if so does the guest router need a diferent ip, and dhcp ip range?
2) Both routers use wpa2, one is channel 1, the other runs on channel 11 is that correct?
3) Then I have enabled access point isolation on the guest router, is that enough to prevent anyone from peering into the business network? or do I need to setup vlans on the main router (and if so how?). Or would that be redundant (vlans seem to add quite a bit of complexity)
4) Do I need to set the wireless mode of the guest router to client, or do I leave it as AP?
5)Then I enabled p2p blocking on both routers, then ideally (bonus) how can I give bandwith priority to the office networks (not the guest wifi).
Anyway some guidance in regards to how to setup this network would be appreciated.