Hi Graham and welcome to the forum.
There are a few of us here who are running 24v C5's.
Hank and I are are both using the C5alive kit ,which has proven very reliable. It uses the standard motor and two car batteries (usually type 063). Plus a cooling fan. The twist grip throttle works well,and reduces the strain on the drive belt and gearbox, should be a standard fit on all C5's !
As a rough rule of thumb, the range of a 24v C5 (which is using two batteries) is equivalent to a normal 12v C5 using one battery. An obvious disadvantage is the loss of ability to carry spare batteries - no space,and weight being the problem !
Advantages are better hill climbing, faster speed and acceleration - but the more power you use,the quicker the range decreases. The increase in speed is not huge, you cant go chasing mopeds !
As for the brakes, changing your front wheel to a steel one is an absolute must. But then for my own peace of mind, I have converted both my 12v and 24v to steel front wheels. The brakes are as adequate for the job on a 24v as they are for the 12v...not brilliant,but they do the job.
Hope this helps.
Karl