Sorry to be vague and I hate sending people to the HSE site but...... Check the HSE guidance - I think there is a free leaflet on grinders. I'd say yes to the stop button and you also need to look at the stopping time of the wheel. Seems far too long to me. New or ( I presume ) old piece of kit?

HSE guidance states only: Stopping devices 43 Means for starting and stopping grinding machines should be clearly visible and marked, placed in a safe position and within easy reach of the operator. On portable machines the control should be located so that there is no risk of accidental starting when the grinder is placed on a flat surface. Our bench grinders have only a start/stop switch at the front. Wheels cannot be slowed down by applying any sort of pressure on the wheels, so the only way to slow down the wheel is to wait until it stops by itself.

You should, NEVER, fit braking to a grinder if the wheel is secured by a single central nut, as is common on a floor mounted, bench mounted, or free standing off hand grinder. If you have a ring flange, which is only normal on large wheels say 300mm dia., then you can look into braking, but, you need to seriously consider the forces on the wheel, and how the jerk and acceleration will affect not only the wheel itself, but, also the balance of the wheel, which at that sort of size is quite important & the mounting of the wheel. To do so is extremely dangerous, please think about the science and engineering behind this before you make such recommendations. I have suggested some of the potential issues above. There is a damn good reason they take so long to run up and stop. There is ONLY a requirement for stopping times on hand fed woodworking machinery, not a grinder. Please refer to HSE ACoP L114 for wood work machine advice, it is actually quite good. The above is part of the reason that HSE don’t mandate a stop time on bonded abrasive machines. If you do not have a no volt release on the grinder, how can you meet PUWER Reg. 14? This must have suitable start & stop buttons , nvr & overload protective function. Does it require an e-stop, down to RA, would it increase the safety, should the stop button simply be an e-stop just to save confusion. This is where the grey area comes in. My interpretation of PUWER & the harmonised standards under the machinery directive is it should have a “proper” starter to be suitable for use in a business. Run down time however is run down time, this must not be altered without serious engineering checks being made, else you may well find yourself in breach of PUWER Reg. 12!

Quite possibly, but it should have a "no volt release" function really to be compliant with PUWER98. A simple on/off switch like a light switch or toggle switch cannot provide this function.