Torque wrenches are not necessary for such things - just tighten them by hand, using a 6 inch ratchet of spanner - but do so with the car on the ground to settle the suspension and allow for any body flexing.
Strut braces are not quite bling - they make a small but definite improvement in initial steering response. However, do not expect any great changes, a strut brace just being a small part of a system, along with poly bushes all round, butt strut, body brace, E46 strut mounts, rear beam bush stiffeners, and improved springs/dampers. A size up on the rear anti-roll bar can also sharpen the steering response.
The Z3 has a very rigid body anyway, and in spite of comments by uninformed motoring writers etc does not suffer from the dreaded scuttle shake - even the best strut brace will not compensate for the deficiencies of the original wishbone bushes, poor or mixed tyres, worn dampers and strut bearings etc. Get the car in excellent condition before spending money on shiny bits and pieces.
See my bit in the 'Articles' section on 'Stiffening a Z3' for more details.
