sounds like a load of bad news on the inter Should of just stuck with the old blue paint.

On the subject of your brakes I think i would stick with the fordson brakes as if you modify them and there is an acident on the road the whole world will fall in on you. They are not approved for use with the tractor. these tractors have been around for a while and as long as the components are good and the tractor is used sensibly i would say you should have no problems with the originals. 60 pounds is not out the way for the cables they used to be 100 plus each! and I think it would cost more than 60 pounds in parts and time to do your conversion.
your tractor is already fitted with a hand brake it is the lever that is behind the belt pulley drive if it works it will lock the transmission solid and the tractor will not move.
The other thing is that if you do decide to go ahead and do the brake conversion is that you will need idependant brakes.
they will help you with steering in muddy conditions with a load on the tractor. The front end may not steer and you can assist it with the left or right brake to point it in the right direction.
Also that tractor has no differential lock so if one wheel starts to spin you loose drive some times you can use the independant brake to slow the spining wheel and get traction back it worked for me once or twice.
have fun catch ya later.
Regards Robert