Strip the upper (whichever one you choose), and bring it back to it's original configuration. Sell it by itself and the parts seperately. You'll get the most money that way.
Which upper? Is the lower a Colt? If so, I'd keep the Colt, just because I'm not into "Franken AR's." The Aimpoint would probably sell faster, just for the fact that it's cheaper than the ACOG, but of course the ACOG would net you more cash. Tough call there. I'd keep the ACOG because tha Aimpoint will be cheaper to replace later on.
Sell the KAC Carry handle and keep the Dieters(sp) because also it will be cheaper to replace the KAC Vert grip
The RRA upper will definiyely be able to bring back to origional spec because of the pront sight. I think the RAS with the ARMS Super extended looks cleaner than the SIR, but the Military version will be a B____ to replace with origional handguards (especially if you cut off the delta ring with a dremel.
The VTlor Stocks are nice. I haven't tried a Magpul one yet, so I don't know which way to go there.
Man, this is tearing me apart just thinking about it. It must be driving you mad.