@AV-Guy 

I will try to address your questions below.

1. If you would like to access the new features of the Deco you plan to buy, it is advisable to use the new Deco as your primary unit and switch the current primary to a secondary unit.

I have included a link below that outlines two different methods for setting up your new Deco as the primary unit. See section "Configuring multiple models of deco units to operate concurrently." 

https://community.tp-link.com/us/home/forum/topic/699816?page=1

2. Regarding which model to purchase, This question presents considerable complexity, as the answer is contingent upon various factors. Please refer to the link below. It contains a chart listing Deco models, along with the corresponding firmware platforms for each hardware version and the features available with each firmware.

https://community.tp-link.com/en/home/forum/topic/631936

In the "Firmware Platforms" column, you will observe a range of versions from 1.5.0 to 1.12.0. The lowest version is 1.5.0, while the highest is 1.12.0.

If you are looking for a Deco model that offers the most software options, the Deco X50 or Deco X55 would be the recommended choices based on the information presented in the chart above.

It is important to note that there are multiple hardware versions of each model. 

If you acquire either of these models with hardware version V1(V1.6) only, your devices will operate on the highest firmware platform, which is 1.12.0, granting you access to all available software options.

Conversely, if you obtain a unit with hardware version V1_1.4.1, it will be on firmware platform 1.11.0, which means that some software options may be unavailable.

Both devices are older and do not support WiFi 6E or WiFi 7. If you are looking for WiFi 7, you may consider the Deco BE95 or BE85. However, please note that both of these options are equipped with the 1.7.0 and 1.8.0 firmware platforms. Consequently, while they are capable of supporting WiFi 7, they offer fewer built-in software options due to being a few firmware platforms behind.

I cannot recommend a specific model for purchase; however, based on my experience assisting in the forums, I would advise caution regarding the XE75 Pro and BE11000 models, as both appear to have significant firmware issues that require attention.

3. See my answer for question #1