1. The main downside of a suretrac LSD is that when the oil gets very hot, the LSD starts to behave more and more like an open diff. This is not a problem on the street. You will never be able to get the gearbox oil hot enough on the street for this to become a problem.
And even on the track, you have to be doing a fair few laps before the oil becomes hot enough to affect the operation of a suretrac LSD.

2. Yes, you can leave DCCD unpowered and it will behave like a cusco tarmac diff. My DCCD 6 speed spends 99.9999% of its time with no power going into the DCCD.

3. At a bare minimum, you will need: Gearbox, front stubs and seals (if using a post 2002 6 speed), clutch fork, 6 speed trans mount, 6 speed gear linkages, 6 speed gearknob, tailshaft and matching ratio R160 diff.
You can re-use your current front hubs and shafts, but because they will be female shafts, you will need stubs from an auto box to convert the front shaft output on the 6 speed from female to male. You also need 5 speed seals put into the 6 speed box as the conversion stubs have a slightly smaller diameter than 6 speed shafts. This is all assuming you use a 2003 or newer 6 speed. 2001/2002 6 speed is PNP with front shafts.
Stubs are around $100 a pair and seals are around $60 brand new from Subaru.

If I remember right, you have a Fidanza flywheel and Exedy Sports Tuff clutch. You can keep them. Its already a good setup so no point in changing it and it will work just fine with a 6 speed.

4. I think there are 4 different ratios to choose from. If you are going DCCD, there are only 2 to choose from. Close ratio (JDM 2003-2005) and long ratio (ADM 2006+) with the taller 5th and 6th.

5. Something like a 2005 DCCD + Plated R180 setup I would estimate to be around $8000-8500 mark drive in drive out

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