i have built 2 different enlarger tables in the past, and both worked really well.
ann's idea of using sawhorses works great. at the same time you buy the sawhorses, buy a door, put the door on the horses, and it'll hold a ton of weight. i had a little durst and a omega d3v on it and no problems.
the second table i built is the one i use now. 4x4 legs cut about 32.25" high, 2 - 2x3 or 2x4 as braces between each of the legs - and 3/4" plywood as the top. i also have a few 2x4 "on edge" running underneath the table to brace the top. i have both the enlargers i mentioned before on it, as well as a huge . auto omega e4.
i have a bunch of other odds n'ends ( read : JUNK ) on the it now too, and this table keeps begging me putting more "stuff" on it
for the vent - if you don't mind something on the side of your house / building that looks "industrial" -you might consider getting one of those grey-metalic louvered (outside window) white grill ( inside window ) window fans that nutone makes. i think i picked one up a while back for $60 - $70.
kind of makes you feel like you are in a photo-factory when you have it up n'running, and is always a conversation piece ...
good luck with your project!