Hi,

For the engine tach first. You have several options. A bluetooth or USB OBD2 is definitely the easiest if you test regular cars. CAN bus RPM is also supported via a plugin. If neither is available you can use an RPM adapter, for example the MSD 8918. There is a video on our youtube channel about that one. Finally you can use the tach of your engine and hold the RPM at a specified set point and press a button. Gear ratio will be calculated and then you get the engine RPM. 

For the flywheel question. Many roller based dynos have heavy rollers and quite high inertia. It relaxes the demand for the brake response when you have cars with a steep torque curve. Unless the flywheel has a low max speed spec, or there are issues with it somehow, I would keep it on. It does no harm. 

Eddy brakes are easier to control than water brakes. Clayton may have added it because their controller was struggling to control the water brake in all situations. 

thanks for the reply  clears  up a few points i was usure on and i  just managed to cnfuse myself more searching.  previous owner added the flywheel in place of the waterbrake as is was apparently troublesome, and leacked  so it was converted to inertia  ( i have a suspicion it froze ) .    more i think about it if the flywheel is easy to keep i'l keep it, if i need the space for the eddy brake it goes.

And then i realised teres a very easy way to get a tach signal out of my cars stand allone managment,   i just use one of the ECU's spare outputs and ask it to output a tach signal,   if i just connect  an opto isolator to that and into turn to the yourdyno box     job done easy