You asked about Downs.. Those who have build them will comment that with the years and "practice", they have an improved product and that their kits are mostly reliable out of the box. http://www.downsmfg.com

I looked at their "37 Ford" PU body before landing on my 39 Studebaker body. I talked to a network of owners with the 37 ford PU version and heard very few "real" complaints.

The recommendation to look one over with 20K miles is a good suggestion. It may be difficult to do. Rodders with keepers tend to to a refurb before the original build gets 20K down the road.

I additionally recommend you go to a large regional show and talk to owners with drivers. The Syracuse nationals is a 6-7K attendance with lots of glass, coming up in July.
Trailer queens generaly stay nice for a lifetime, given their anti road policy. TQ owners are not usually the builders either and have minimal reference to the real isssues of building/owning glass. Look at and talk with the the driver keepers and ask those owners the questions.

Glass will eventually give you some trouble if you drive it. Most any notable glass builder nowdays will have a reliable product if they have any scale to their business. I believe the days of chopped - thin paneled - flimsey "kit cars" are about gone. The real issue is panel fitment and consistency in the field of the flat panels.

Also, If you want to save a nickel, some of the Downs bodies can be built on existing platforms, I believe they still use the Chevy S-10/ Sonoma frames. speaking of frames, that's a whold other issue - lots of em out there and all have their own issues among builders.

good luck with all that....