There are 2 collectors from SPD one that is 4 individual tubes and one that has been formed on the outside to fit on to your 4 tubes instead of slide over the 4 tubes. The one that slides is either just partially welded or some guys build tabs to allow you to slide it off.
The 2 collectors are called merged collector and formed collector.
Asl far as doing one whole tube at a time it just depends on how difficult they are to snake through whatever you have to clear. I usually try to build the tube that can rally only go a certain path. The others are more flexible. Sometime you have to build or change tubes a few times to get everything to fit. This where you pick up time with experience and being able to use the scrap bends. You can tell a guy has experience building headers when he has one weld seam per bend and straight section. Not a weld seam on both ends of a straight section and multiple seams in one single sweep that does not change direction mid bend.
I would build your first few sets out of steel before, you step up to stainless. Stainless is pretty expensive. SPD makes really nice flanges that are CNC and not laser cut. They also use the absolute best materials so you welds look even better. 304 stainless is almost 3 times the price of mild steel and 321 stainless is about double the price of 308. .
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