Just had one installed.
Do not let anyone install as you mentioned.
You need a concentric vent.
In my area not one of the 4 a/c contractors that quoted me had installed a 95% furnace. By chance the contractor I used went to a class given by the a/c distributor the night before. Please go over the specification with the contractor. there are certian requirements base on unit vent/combustion sizes. this may vary with manufacture and furnace. Some require minimum/maximun number of pvc elbows to balance system base on condition and lenght. You can go thru the same vent hole but you will need a new vent assembly to pass thru roof and is designed for your concentric vent. It should be part of the kit.
Use this link to see one. Play video.

http://www.alpinehomeair.com/viewproduct.cfm/productID/453056394/p/Goodman_DCVK_20

Wow. I'm almost ready to tackle it myself to get it right. I like your idea of going over the specs w/ the contractor.. but these sales folks don't want to take the time to do so. I was all set to go with Carrier due to good ductwork description, but got the contract and see the wrong numbers on the hardware pieces, i.e. says 95% efficient furnace but only 80% furnace listed. Might just be a simple mistake but I guess I expect it to be right. Thanks for your note, RC!

poppie,

I told them up front I was going to be there to check every item against the contract and I did. I pulled all labels of the boxes and kept for my files. Take thier equitment numbers on the contract and go check them out on the manucafutres site. just don't go on what they say. Check it out youself and ask questions. make sure it all written in the contract. Don't worry if ir drives them crazy. just remember they will only give you whats on the contract.
In a few hours I will be away from my computer
till thursday. Oh, Yeh! Relax time.

Thanks RC... enjoy the vaca... yes, I've learned that after the fact, the only thing that matters is, "What does the contract say?" Anything else is hype!