Micro 4/3 wide angle options

To echo what others have said... The 9-18 is currently on sale everywhere at $600 rather than $700, still not cheap, just giving you a heads up... Last time it went on sale like that was last summer. You can find it slightly cheaper ($50-75) used or new but imported, but it hardly seems worth it.

Beyond that there isn't much that's wider AND cheaper... You could try defishing a $200-250 Samyang/Bower 7.5 FE, which I quite enjoy, but it'd be iffy for something like real estate interiors where precise composition is key. Could work if you're mostly publishing online tho... Stitching in post seems like a surer bet otherwise.

The Olympus 12mm was on sale yesterday at Oly's refurb outlet for $480 ($640 - 25% SPRING promo), in both black and silver, and it sold out in under an hour. I've seen the 9-18 there for around $400 before discounts, but it's an even rarer sight.

You basically have to stalk that page day and night if you wanna score any of Oly's refurb deals, they tend to undercut even the used market tho. I missed the 12mm twice and got real lucky on president's day as the refurb price was lower then ($480) and the holiday discount brought it to $384.

Edit: Renting wouldn't be a bad idea, as suggested, I almost rented the two m4/3 UWAs when I was trying to decide between them. Dunno why you wouldn't use a tripod for this kinda thing tho, but I imagine flash would be a issue unless you start doing more specialized off camera lightning.