Question 1: If you will need to emulate ~253 concurrent users at peak then yes you will need the team plan to reach this level of concurrency. Without knowing full details of your requirements, it’d probably advisable to test a bit beyond 253 to ensure there is some room for exceeding your targets in this environment. This should give your client some comfort knowing that they will perform beyond peak load.
Question 2: You can set custom user agents with the user agent option for the entire test: Options reference
Alternatively - the cloud also has some other methods for whitelisting which you may want to explore: k6 Cloud Documentation
If whitelisting user agents does not work, we do offer static IPs if necessary - those carry an additional cost starting at $30/IP/month. I’d recommend to start with the User Agent method though since it’s built in/ easy to set / free.