Hi diggidy713,
Floor models are generally replaced on a regular basis and generally haven’t had enough useage to make any meaningful or practical difference in the durability or useful life of a good quality mattress so if you are OK with the idea that other people have been lying on the mattress in the store then if a floor model has the same warranty as a new mattress then there would be very little disadvantage to buying it if a floor model happens to be available.
Mattress warranties only cover defects in a mattress which are relatively rare if a mattress uses good quality and durable materials (which Beloit certainly uses) and they don’t cover the gradual loss of comfort and support which is the result of the gradual foam softening and breakdown of materials over time which is the main reason that most people would need to replace their mattress.
Having said that … a foundation or a box spring (they are different products) that doesn’t provide a suitable support surface for a specific mattress can certainly affect and reduce the durability and useful life of a mattress and affect how long it will be before you need to replace it. There is more information and some guidelines about the different type of support systems (bedframes and foundations or platform beds) that are generally suitable for different types of mattresses and some examples of each of them in post #1 here but different manufacturers can have slightly different requirements that may be specific to their mattresses.
I don’t know the specifics of your split queen foundation but whether to use it or not would really depend on its age and condition and on your own risk tolerance and the level of risk you are willing to take. If you are more of a risk taker and are comfortable with a higher risk and your foundation is still relatively new and still in “like new” condition and is suitable for your mattress then it may be worth taking the risk but if you are someone that tends more towards the “better safe than sorry” end of the risk tolerance range then I would purchase their foundation as well.
You are certainly looking at some great quality/value choices and I’m looking forward to finding out what you end up deciding
Phoenix