You have posed a number of questions. Here is some information which should be helpful. Migration Assistant is very easy to use. Be sure to read and follow its instructions as you progress. Basically, the process is as follows.
You first begin by starting the process of setting up your new computer. You will be asked if you wish to transfer files. Respond YES and follow the instructions you will be given.
You will be told to turn both computers on and activate Migration Assistant in the old computer.
The computers should be connected with a USB cable to facilitate the transfer. (As I said Wi-Fi is simply not suited for a transfer of so much data and corruption often results.)
You will be given several options as to what to transfer. You will likely want to transfer your settings and preferences.
You can transfer all or none of your files or apps. You do not have an option to select certain files or apps to transfer. If you wish to do that, then do so manually. Some people prefer to download and install new apps in the new computer. In either case, you may have to reregister or upgrade some older apps. Note that if you are running any old 32 bit apps in the old Mac, they will not function in the new Mac and 64 bit versions will have to be newly installed there. And there can be a cost for the upgrade in some cases.
The old computer's operating system will not be transferred. No data from the old computer is erased.
Allow for up to an hour for the transfer process to complete once you start it.
It should set up the new computer the same as the old one.
As to the cable you need, get one with the right connecters on each end that match the USB ports on each computer. If you check Apple's support sales site, it has some excellent pictures to show you what is needed. You ask about the wireless keyboard cable. My answer is does the cable have the right connecters that you need? It's a simple matter to look at it and see. In the alternative an Ethernet cable will work, but it will be slower.
When completed you will have two user accounts in the computer. You may either delete one of them or select one to use.
Just follow the instructions and everything should go just fine.