If you are cash and resource strapped, then I would NOT recommend using MongoDB. Seriously, in terms of cash, Postgres can cost you nothing; if you are resource strapped - use Postgres, there are a heck of a lot more people who know it than MongoDB. If you can't setup replication, then IMO you have some seriously larger underlying problems with your startup.
Don't forget that a lot of people have realized that MongoDB is NOT a good fit for what they want to do. Then they have a resourcing problem - a big one.
MongoDB costs you nothing as well. And there are plenty of people who know MongoDB as well as 10gen who can provide official support if needed. It is quite a popular database you know.
And there are plenty of people who are switching away from PostgreSQL and other SQL databases to MongoDB. Again it is quite a popular database (hence the huge amounts of cash they seem to be easily raising).
Who are you to lecture others on what is/is not important for their needs ?