I prefer 24v as it can go farther, and is sometimes brighter. You're going to find real quick that you want more LED resolution than the 8-bit decoders can provide. The stepping at the low end is killer. A lot of decoders and LED tape are kind of cheap (because they are from Amazon), so you may want to get a couple extra to prepare for failures. You'll also want to find a way to disperse the heat of the LED Tape, that will kill your strips quick. Here's my go-to list of gear for this type of thing.

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Power supply
https://www.mouser.com/ProductDetail/709-HLG-600H-24A

16-bit Decoder
https://www.amazon.com/gp/product/B075FHJM35/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

LED Tape
https://www.amazon.com/gp/product/B01MYN0HUU/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Aluminum Enclosure
https://www.amazon.com/gp/product/B01DM7FZ70/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

1. I've found that generic decoders from Amazon work just fine. You might find that troubleshooting generic devices to be difficult due to lack of documentation, but generally LED DMX decoders are pretty robust.
2. It depends on the decoder. There are some that are multi channel decoders. This one for example can control 8 individual strips.
3. As long as your power supply can match the amperage used by the sum of the amperage your devices and matches the voltage required, you can put multiple devices on one power supply. The conventional wisdom is to ensure that your power supply can do at least 10% more amperage than your devices need. Also keep in mind that if your one power supply fails, all of your devices will fail.

Light bars are nice because they are generally more versatile than LED strips. For example, the beam spread for RGB5050s (if I recall correctly) is about 5 degrees, whereas most LED bars are much washier. If you're just aiming for flash and trash, I would start with the decoders and go from there.

Hope that helps!