HE Tubes Through Unheated Attic

Shopping for a replacement for a 26 year old Bryant gas downflow furnace.  The existing unit is of the spark pilot / inducer type so probably 80% efficiency.  Weighing the choices of staying with something like that, getting a 2 stage, or variable, or some sort of high efficiency whose added cost is somewhat offset by some gas company money and the tax credit.  Had two vendors give me estimates.  One does Carrier (or Payne); the other Lennox.



The machine is in an interior closet with the water heater.  Lennox guy said he strongly discourages installing an HE in that space due to the PVC venting having to go through an unheated attic / crawlspace.  He said it would lead to condensation and possible freeze up.  Condensation seems an odd issue to bring up since I thought that's what those things do and it just drains back and goes down the drain just like the condensation from within the furnace.  But I suppose freeze up could be an issue?  The unheated attic space it's passing through is probably under 4 ft.



I was already having concerns about future repairs on a complex HE furnace wiping out savings gained.



Lennox dealer guy also dismissed two stage and recommended their model that is fully variable speed.



As a separate question I kind of like the way the old furnace worked with a spark-lit pilot which has to get proven before turning on the main gas.  Does that exist anymore?  One drawback to the Lennox models to me is the hot surface ignition and while this dealer offers a long warranty including labor and a promise of 2 hr service day or night, I understand the HSI thing, even the improved versions, are still an expected failure point just like a light bulb that will burn out.  Or should this not be a concern of mine.   The Carrier dealer's proposal didn't include specific model numbers so I don't know about those.