Actually some clubs are now running reliability rides but called “early sportives” for 3x the price and selling them out, whereas an ordinary reliability costs about £4
I did the rebranded Catfood CC reliabilty ride on 1st Feb (it’s now called the Hell of the Ashdown), £15 for a 100km route. Admittedly very nice and well signposted and well thought out but the feed stations were distinctly lacking – bananas and water at the first, tea and coffee at the second and that was it. Still, it sold out very quickly, there were 1000 riders doing it!
As to the OP, unless stated otherwise, you can do an audax on more or less whatever bike you want. Some are a bit more strict and will specify mudguards.