[Renze]

I have no idea what kind of trailers are usual in your part of the world, but look at the overall build:
I am in the trailer business myself, but if you buy a 5 ton trailer over here, you get torsion axles with cable brakes, and wimpy chinese tires. If you buy a 7 ton trailer you get air suspension, cam brakes, automatic brake force adjusters, ABS and premium brand truck tires. It's not just the 2 tons of load capacity, its a whole different class of trailer.

I would definately recommend having brakes on all axles. You might save some $$ on not having brakes on all axles, but the cost of wear on overloaded truck brakes, will soon be higher than the cost of brakes on that other trailer axle. Get a little extra brake capacity, its safer, and with better technical margins, the brakes in total, wear less.
The brakes on your trucks front axle, usually are a lot more expensive to replace than trailer brakes too.