There are a lot of good cameras out there for cheap. I have installed a couple dozen Amcrest cameras that were in-expensive. I suggest going with POE cameras then you don't need power and you can just run ethernet cable to each one to power it from an NVR with POE. As far as security goes, very few individual devices have much security to them at all other than passwords many times in an un-encrypted connection as well. Best practice imo would be to have the cameras hooked up to an NVR, then your stream to your phone comes from your NVR containing all of the feeds in one stream instead of individually streaming from each camera. You really should be concerned about perimeter security much more, so if you have a firewall lock it down, and find out what ports you need to connect on a secure connection to the NVR make a difficult password and only allow those ports to the NVR, block all other WAN access to the devices.
Really every IOT item and internet device in homes should be looked at. See what ports are necessary for the operation that you need for them and open those ports just for them, then generic ports for browsing like 80 and 443 and some mail and DNS stuff may or may not be necessary. I realize I ramble so long story short, for cheap that work well with 2 way audio look into Amcrest and their NVRs, decent bang for the buck.
Also if you have a POE switch/ NVR with POE and put a battery backup on it, then it would be a backup for all of the cameras
