First off, welcome.
second, assuming you know all you need about selection of sensors, you came here looking for electronics, specificaly help with selectin of both the input devies and the jumbo displays that can be seen from 20 feet away.
how fast are you looking for parts ?
max6675 is a K type, serial input and resolves to 0.25 deg C (has output in either C or F )
costs less than $10 on e-bay.
I see it on e-bay, but that can take weeks to get. most come with K-type, but I would assume that for a trade show, you want T-Type for the sensitivity.
it is probably the easiest way to get readings into the system. least work on your part.
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use any board, an UNO or NANO would work fine.
what you want is not really that hard, any board has the capabilities.
if you just want to buy your way out.....
visit Cross Roads Electronics
I rarely recomend places, but I believe Crossroads offers pre-made boards with the drivers you need and can drive your jumbo numbers without you having to solder.
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DISPLAYS;
I have to leave this to others to offer JUMBO displays, not so large as a high school score board....
the other reason crossroads came to mind is that he is one of the guys how knows where to get displays.