UK user here. Some info about what I have setup. No right or wrong answer, the info below might just give you some lines of enquiry.
2 external cameras:
I too am a user of BlueIris. Solid product. I have 7 cameras hooked up to it and I generally keep that solution separate to ST. The only exception there would be the fact that I use one of the camera feeds on an ActionTiles panel via a tablet. That said other users are using BI to detect motion and then carry out actions within ST.
4 door sensors
I use a combination of ST, Orvibo and Xiaomi. As discussed above ST sensors give you temp and vibration but if you just want basic open/closed detection and you property is not huge then go with the Xiaomi or Orvibo. Solid bits of kit and shouldn't cost you much more than a tenner each.
External Sounder (alarm style)
Don't use one. I have a separate old school alarm panel. I use ST to control the home environment and provide information when I'm away, for example: When I'm away the heating goes down to 10, lighting changes so that it comes on (as opposed to motion based when at home), garage door is diabled, etc, etc. I rely on the alarm to scare off Burglars and keep neighbours swearing.
Control of heating
I've got a 3 zone heating system. All controlled using a community developed handler. I have 3 battery powered z-wave thermostats that cost me about £40 each set on eBay (search Secure or Horstmann).
Pet immune type sensors
Not sure here, haven't researched it heavily enough. Question here is you have door/window sensors on all entry/exit points then do you need to use motion sensors for security. Note that my alarm system has wireless pet immune IR sensors on it.
Couple of other points.
CoRE - Get this installed and have a read around it. It's an essential component of the ST toolkit. If a Smart App doesn't exist for your requirement then CoRE can do it. This is a very clever if X then Y else Z rule engine. Quick use case: I have a TKB z-wave metering socket on the tumble dryer. CoRE is setup with the following rule: When power drops below 10W for more than 120 seconds send a push notification stating "Tumble Dryer has finished". It's the little things that matter!
This community and search is your friend. The amount of co-operation and willing to assist from members is awesome.
If you've got nothing better to do grab a drink and starting reading the FAQ. Lots of useful info in there!