You got it buddy...
1) You can swap it out easily, some "kid" would LOVE to swap his OEM intake for it. It's not a bad performer based on my research but there are better intakes for the G.
2) Don't buy your warranty from a dealer, find our link to Warranty Warehouse (on the right side of the page, hit refresh if it doesn't show up). I have used them, they're awesome, and offer the SAME warranty your dealer will sell you (minus the fat dealer markup).
3) $26K out the door. No tax (out of state), title / license will be paid in AZ and will be probably $500 or so.
Go to
http://www.kbb.com, I'd recommend you offer Kelly Blue Book Trade-in value and work from there. Don't go anywhere NEAR retail, as it's a private sale. Look at autotrader.com for values as well.
If it's PERFECT, I'd pay no more than $23K. Trade-in value is between $17-21K, depending on what he's buying.