My professional opinion and experience I think you'll be fine using K#. If you have the ability to do some reporting on the transactions per day that might give you a better idea of what you're working with per user. For instance every user processes 10 transactions per minute or every minute there are 30k transactions.

Once you figure that out you should be able to beef up your server to handle the processing, create some indexes on the database etc. Another thing you could do in your test environment, turn on debugging in Kentico. This will allow you to view the queries and macros that are run for each page load and how long they might take. Then you can go to your pages, perform some transactions and get an idea of what queries and macros might cause you problems.