Multiple passes aren't a good idea with MIG, but will work. I generally weave a bit with a thicker wire after making deep bevels when I need to weld thick material. The more molden shit you put in a seam, the hotter it tends to get.
The parts I've made in sch40 haven't cracked so far. The easiest and cheapest thing to start with is the sinktrap manifold. The stock cast 4-1 manifolds for 90-93 B18's and 88-95 non-vtec D's are great for that. After the practice you get from that, you can step up to a 4-2-1 or a log. If you have fast grinding tools, a shorty/elf manifold would be good. FYI you can always sell the manifolds for more than the cost of the fittings, how much more depends on aesthetics...