Q1 I would backfill with a concrete pad to ground or at least a fair sized pad under each leg of the table/stand. Wooden floor will move around over time plus vibrate

Q2: with a typical 200mm bearing spacing you will get a cutting area of about 400mm square and about 150mm Z travel. The Z travel is a good amount for a typical general router but the cutting area might be on the small side - depends what you need to cut , size wise.
The rail lengths need to be similar to the ballscrew threaded part length plus a bit spare depending on the detail of your design. You need to sketch it out to check for the correct length. Be aware you can trim them to length with a angle grinder and a thin inox blade.

Q3: what's in the box? Linear rail carriages? I've not found someone who will supply rail to match someone else's carriages.
In the past I used bearings for guides similar to your sketch but they were not the best and needed adjustment and TLC. Supported round rail will be much better or most people end up with proper linear as this is a real fit and forget solution. Get a quote from Fred at BST automation on Aliexpress.

I've looked at the banggood linear profile rail but they only sell 400mm at the moment. I've asked about longer lengths and they are looking into it later this year so they say.
Quality unknown but i was interested to try some of the longer lengths and test it out (need to know if they are properly hardened etc) and compare to the branded stuff I have now.