Typically the old style battery "testers" with the sharp prongs to interface with lead terminals take a current draw in the high hundreds or thousands of amps for a second or two, whilst indicating the current through a robust low resistance. This may prove the CCA (a laboratory figure in any case) but probably takes a lump out of a new batteries life.

Low internal resistance indicates a batteries ability to deliver high currents so is important for a starter battery but its not a figure that's often used. For longevity battery capacity measured in amp hours (Ah) is the best guide. High capacity means a) an easy starting car puts little percentage drain on it, and b) as capacity naturally reduces with age you have a usable capacity left for longer.

My Roadster is factory fitted with a generic type 075 battery with C20 rate capacity of 60Ah meaning it should provide when new 3 amps for 20 hours.

45Ah sounds a little low for such an engine but it may be you don't have space for larger capacity. As a rule I would always fit the largest capacity that can physically be accomodated. The price not being proportionate.