For this journey I would definitely take a cab. You can pick one up right outside St Pancras, and it will drop you off right outside Liverpool Street station where it's a simple matter of taking the escalator down to the actual concourse and picking up your ticket. I'd estimate between £10 and £15.
Definitely sufficient. At that time, the tube is likely to be running normally so I'd probably not take a taxi for the return journey unless you happen to have lots of luggage.