Hello -- Some of these questions will ultimately require you to answer, but we can certainly help you think through it a bit. Here's what I'd offer with regards to your questions:

• Body impressions: In general, the thicker the cushioning, the more likely it has a weak link that will lead to body impressions. A weak link is often something like lower-density foam up near the surface. Sears Canada's website doesn't provide the specs on all the layers of this mattress, but if you can find them, post them here and we can help you assess it.
• Support: The key here is to assess whether your spine is in proper alignment when you're laying on the bed in your typical sleep position(s).
• Motion isolation: We rate this bed an "A" for motion isolation. The pocket coils and heavy padding are helpful for this.
• Temperature: This bed should have roughly neutral temperature. Simplistically, it doesn't have memory foam, which can get a little warm for some people. If you go with the Plush version, you'll probably sink further into the bed, meaning less of your body's surface area will be exposed to the air, which could make it feel slightly warmer. But I wouldn't expect this to be an issue unless you have commonly experienced problems with sleeping hot in the past.
• Firm vs. Plush: This depends completely on your preference as well as other personal factors like what position(s) you sleep in, etc. If you're a side sleeper, the firm might not provide proper spinal alignment or pressure relief. If you're a stomach sleeper, the plush probably won't provide proper back support. If you're a back sleeper, you can probably go either way, but may have a clear preference from a comfort standpoint.