Based on the official Richard Mueller novelisation we can see that Venkman really likes the building but that that Egon genuinely thinks that it's a rat trap:
Venkman thought it was perfect, but he had no intention of tipping his
hand too soon. “This might do . . . I don’t know. It just seems kind
of pricey for a unique fixer-upper opportunity, don’t you think? We’re
trying to keep our costs down. You know how it is when you’re starting
a new company.”
“Yes, I know. What are you calling your business?”
“Ghostbusters,” Venkman said coolly. The name had come to him in the
middle of the night, a flash of inspiration, and he was rather proud
“Oh, well, this place is perfect for it.”
“Perfect?” Spengler echoed sarcastically. “It needs a new floor,
rats have been gnawing the wiring, the plumbing’s shot, it looks like
hell, probably in violation of at least a dozen building codes, and
the neighborhood’s a demilitarized zone. I think . . .”
This is flatly contradicted by the films' other novelisation (by Larry Milne) in which Spengler's less-than-enthusiastic report is more of a ruse in response to Venkman's silent plea;
Venkman signals with his eyebrows to Spengler.
At this rate they're going to have to pay top dollar. Spengler comes
to the rescue.
'Miss, have you taken into account this building's grave defects?' he
says in his funereal voice, and begins to list them. 'Beam structure
faulty, wiring substandard, floors subsiding, dry rot in the
wood¬work, plumbing disconnected
Venkman nods happily. This is more like it.
In the screenplay, his wording is slightly different but it's very clear that the building will require a hell of a lot of work to bring it to a usable state (as evidenced by the "moving in" scene) and that it's totally unsuitable for their needs.
In the film version, Venkman asks him for his opinion (in front of the realtor), knowing that he won't be complementary about the building:
PETER : It just seems a little pricey for a unique fixer-upper
opportunity, that's all. What do you think, Egon?
EGON : I think this building should be condemned. There's serious metal fatigue in all the load-bearing members, the wiring is
substandard, it's completely inadequate for our power needs, and the
neighborhood is like a demilitarized zone.