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Whether to use VSS depends on whether you really need an additional core chassis for redundancy (vs. single chassis with every other component being redundant) and whether you're going to run your 3850 edge as L2 or L3.
What VSS adds is redundancy of a 2nd chassis, which adds somewhat to overall reliability. However, two chassis running L3 adds even a little more to overall reliability as you're adding redundancy of a second IOS.
So, if you believe you need the additional redundancy of a 2nd chassis, if your edge will be L2, I would go with VSS. If your edge will be L3, don't use VSS.
For your 2900s, if using VSS, Etherchannel them. If not using VSS, using routed links. For either, your choice of routing protocol.
PS:
BTW, 6500s are going away, so if buying new, like Leo, I would suggest considering its replacement, the 6807-XL. (Depending on your core device needs, if buying new, you might also consider the 6880 or 4500 series.)
