Make sure that you don't just have an agreement on what everyone wants. It is inevitable that either someone will not meet the house's expectations or will interpret the expectations in a different way. You also need some sort of a process for resolving whatever issues may come up.
Agreed, its important to be realistic. With the hot tub, I made a detailed list of all expected expenses, plus margin for unexpected - I think I heard that projects usually cost about 20% more than the estimate. It ended up costing quite a bit more in practice, so I checked with people to see if they were okay with the upward adjustment, and then asked for a max that they were willing to spend so that I could continue working on it without continued asking at every point. We were able to make it happen for under the max.
Another trick with group houses...
I've lived in several intentional communities and have been one of the creators of two that I've lived in. After recently securing Zendo, I wrote up some tips to a friend who is thinking about starting another Berkeley house, which Nisan pointed out might be valuable to share with the community at large, so here it is guys! These tips apply most to shared rental places, as opposed to bought property, although the stuff about vision applies to pretty much any joint venture that people embark on with shared leadership, such as group housing, event planning, and start-ups. The part about using padmapper.com and acting quickly applies a lot to Berkeley in particular, because the housing market in this city is especially messed up with rent control and thus finding good places is particularly difficult. We found our place over a month in advance of move in, and it was about $200-300 cheaper/room than similar places in the vicinity/class we were looking at that showed up during the same timeframe. Using these techniques is how we got it — I called within an hour of the posting thanks to being on email when I got the padmapper alert, set up an appointment to see the property manager right away, and while I was there in person, another person made her an offer of a downpayment on the spot. I'm pretty certain that its because of my pro-activeness and handing her a big stack of rental agreements and credit reports that we got this place rather than the several other interested parties. Many thanks to Kevin Fischer, Louie Helm, and Eliezer Yudkowsky for helping me with the rental search and acquisition information!
Tips: