My first advice to John Haskey is to either attend a few street organ
festivals or watch some on YouTube.  He can then appreciate the variety
of carts either available or home-built and witness them in use, both
for the act of playing the organ and, just as importantly, moving them
from place to place.

To answer his specific points, yes, you must have a brake, but this
doesn't need to be some fancy tread brake acting directly on the
wheels.  What I use, and most others do too, is a simple 'chock' that
sits on the ground between the two wheels preventing them from moving.
This is easily stored in your music drawer under the cart.

Secondly, suspension.  Again, yes, you must have suspension, although
many people with home-built carts use pneumatic tyred wheels without
a separate suspension system.  To be authentic, you need wooden spoked
wheels with steel rims, although a hard rubber tyre is quieter in use.

There is no requirement to disable the suspension whilst cranking.
Even if you have a low-frequency suspension with a large travel, such
as the carts made by Raffin in Germany, the organ will simply sway a
bit during cranking and this is not at all off-putting or a hindrance
to playing.

One aspect often forgotten by home builders is the balance and
wheelbase of the cart assembly.  Do not make the wheelbase too great.
Ensure that the centre of gravity of the organ and cart combination
lies forward of the rear axle by somewhere around 1/4 to 1/3 of the
wheelbase.  This will aid steering the cart, which is done by pushing
down on the handle to reduce the load on the front axle.

Another tip is to make the drawer under the organ as large as possible
so you can store all your music, plus everything else you may need
during a day's playing, such as drink and sandwiches!

It's now less than a month to go until I will be playing my 26-key
Bacigalupo at the Llandrindod Wells Victorian Festival in mid-Wales.

Best wishes,
Nicholas Simons, GB