I like sturdy exhaust manifolds for turbochargers. If you can't brace or support your turbo head unit outside of the exhaust tubing/piping itself, you need a stout manifold design. I chose stainless 1.25" 304 stainless pipe in schedule 10 (note "pipe" is measured by ID, "Tube" is measured by OD). The reasons being:
1) 1.25 sch 10 pipe has almost -exactly- the right actual OD for the exhaust ports.
2) It's much better than mild steel, but much more affordable than more exotic (though better suited) steels.
3) The actual ID is slightly larger than the stock exhaust manifold
4) It's readily available in straight AND cast elbows (weld-el's they are sometimes called)
5) It's strong as hell, maintains good strength at high temperatures, and very easy to weld
Their are some drawbacks to using the stuff, such as the higher cost and the way it likes to shrink & expand in reaction to temperature, but the former isn't so bad, and the later you compensate for when you build. Or clearance for after you make the mistake, heheh.
Along with the pipe and tube we used a section of 3/8" stainless plate to serve as a 'one piece' exhaust flange. This was done over individual stock-like primary clamps to save time and add a little more strength. Remember that a turbo gets pretty heavy hanging from just the exhaust, and metal gets a lot weaker when it gets really hot. It's a bit overkill, but it's easier to remove metal than to add it after the fact. To complete the manifold I picked up a cast-stainless 3/8" T25 exhaust flange & gasket off ebay somewhere.
Turbo choices for many people are a personal matter. Some go by hype, some go by whatever everyone else does, and some I swear close their eyes and pick the biggest/cheapest one they can find that might fit. I do math, and pick a compressor that works well with the aplication and that has room to grow in the power range should I feel the ned to do so, and then try everything I can to get the ideal unit to fit. You have lots of choices with good turbo's these days: wastegate type, wheel construction, bearing type, turbine A/R sizing, and more. I went with the Garrett GT28RS (aka the Disco Potato). This turbo is just right on the compressor flow, and is the benchmark for compact & quick spooling ball bearing turbos. I also opted for the larger A/R exhaust housing, as that optimizes your powerband for higher RPM peak power. There is a give and take with sizing A/R ratios this way, but you can go to Garrett's web site and read about it there if you're interested... it's too verbose to put in here. Simply put, I didn't -want- anymore low end power on the Concours than it already had: It just really fell on it's face after 6K rpm, and that disappointed me.
Anyway, constuction of the manifold was by far the hardest and most time consuming part. The condensed version runs about like this: Take everything apart (fairing and exhaust), and support the turbo where you want it to fit somehow. Then construct your flange and bolt it to the head. For my that meant cutting 4, 3" or so sections of the straight pipe and cutting holes in the flange plate for it to go through & holes for the studs to go through. Tack the pipes at the depth through the flange that lets you bolt it to the head, test the fit, then weld the pipes. Once the (mostly) finished flange is bolted up, then you cut up weld-els and pipe until you get them all to come into the collector flange where you need them. Bandsaws are fantastic for this, though plasma cutters and 'chop saws' work as well. This is probably the hardest part for most people, and why "kits" are usually sold more often than made. Once you get all your pieces lined up/tacked in place & it holds the turbo where you want, you break it all apart, clean up the pieces and weld it all together (see photo for pre-welded manifold).
Once everything is welded together and the turbo is hanging on its own, you construct the downpipe and connect it to the rest of your exhaust. Making pipe fit can be a headache, and it will almost never go exactly the way you wanted it to go, but you get it there none the less. Weld in an oxygen sensor bung where you can and clean up the mess. You're ready to start plumbing fuel, air, and oil.