You do not have to file a Form 1 (and pay for the $200 tax stamp) with BATFE to make your BREN. The Form 1 is for "making" an NFA weapon such as a SBR, SBS or silencer. Notice the term "making". The Form 1 allows you to "make" one (1) NFA weapon that is listed on the form. Making is for your personal use. Manufacturing is a completely different beast and it requires licenses from BATFE. These licenses allow you to "Manufacture" as many wepons as you want and they can be sold like any other firearm.

Regarding your BREN build the only things you have to make sure of is that your finished weapon is semi-auto only and it complies with 922r in order to make a "non-sporting semi-auto from imported parts". There are twenty (20) imported parts as determined by the powers that be listed in 922r. You can make a semi-auto rifle from an imported parts kit as long as there are NO MORE than ten (10) of the imported parts used in making the rifle. That means you must replace enough imported parts with US made parts in your build to bring the total of foreign parts down to 10 or less. Some firearms do not have all of the 20 parts listed in 922r. An FAL, for instance, only needs seven (7) US made parts because there are only 17 of the 922r parts in the firearm to begin with. The BREN has no trunion because it is an integral part of the receiver and it is not a separate part.

A listing of the 922r parts for a number of firearms can be found here:
Section 922(r) Parts Count

Unfortunately, the BREN is not one of the firearms listed.

I hope this helps.

Chris
aka CGinCT