Gart,
I have two MGS Bren kits.... through a Weaponeer group buy, I got the semi auto conversion parts kit for one of them.

Left to do is the blocking of the receiver and the welding like you mention.

You do NOT need to file ATF paperwork to make your own rifle. A semi auto design is allowable....

The $200 tax you are referring to, is based on a transfer of a NFA weapon I believe. As you probably know, post 1986 manufacture of a machine gun is prohibited by the general public. New manufacture machine guns are only as post 86 dealer samples transferable to other dealers / manufacturers and law enforcement.

I would stick to a proven Semi Auto design, and insure that it is engraved somewhere that it is Semi Auto only and has a legible serial number. This is to keep you outta trouble if you are stopped and asked about it.

I know that some folks keep the ATF letter of the proven Semi Auto design with their rifles, just in case they are questioned.

The big pearl of wisdom to share, is to convert as much of the rifle as possible to the semi auto (bolts and FCG's)... and weld in a blocking bar, before welding up the original receiver. As you may know, the torch or cut pcs are just interesting items until you start fabing... then they could become "intent" to manufacture.

As to selling the semi auto rifle..... it is permissable to sell your own manufactured rifle. However, it is a grey area due to the fact that you don't have a manufacturing license... I would intend to keep the rifle after you finish it, if I were you. Besides, that rifle would have questionable value, onless you are a recognized individual or corporation building such devices.

If you are doing a novel Semi- Auto build, then you may want to submit that design for proving, to the ATF. This would be a prudent measure.

These are just my opinions, and I am sure others will chime in. It is always best to follow up with your research, since I may be in error, and don't have a law license, and am not an expert in regulations governing all ATF and local/ state regs.

I look forward to your build! Ask if you have further questions!!