Newbie question - Biltong Box Project
Is my idea sensible and achievable (technically)?


Yep, sure is. It's not really much different to a DIY Reflow Toaster Oven.


  I know that I will need a 240V relay to control the heater. Is this safe and reliable? ** This is my biggest concern. What can you recommend for this - there are so many available on the market.


I would suggest an SSR instead of a mechanical relay. Easier to drive, and less electrical noise.


  I think I will also need 3 more additional relays for the 2no fans and LED strip light. Is this the best way to control the 12V fans and LED light?


No. Use MOSFETs for those. You can drive those with PWM to change the speed / brightness.


  Can I control the speed of the proposed 12V fans? And how is this achieved?


Yes - see above.

As far as control goes you may want to investigate PID as your calculation method.