Hijet Brakes, need help!! :)

I just bought a banger of a Daihatsu Hijet for £80. The problem is the rear brake pipes have rusted so its lost pressure completely. I had my mechanic friend take a look at it and he said the compensator valve is seized as well. does anyone know how big a job like this is to fix and how much roughly it would cost? The van doesn't look too great underneath, shes an old girl, so its not worth spending a fortune on but i have got £150 sunk into it as i had to get it towed.

any help would be much appreciated thanks,

Minky