"Oh! will it be so bad to mix down with Monitors in a mostly untreated room?......"
Well, yes. Although like everything else it's a matter of degree - mixing down with monitors in a MOSTLY untreated room will be slightly better than mixing down with monitors in a COMPLETELY untreated room, and so on. It's all a matter of how much money and time (eg, learning about it) you want to put into it.
It is however true that if you spend $1000 on fancy monitors and set them up in an space that is completely or largely untreated, you are wasting money. You would probably get the same result with $400 monitors and then be able to buy yourself a fancy soft synth. This is just the way it works. There is nothing WRONG with not being able to completely treat your recording/mixing space the way a professional setup would be. That's where most people are at - it is just financially or physically impossible. It is important however to understand the tradeoffs involved and plan appropriately - eg, spending $1000 is not likely going to "make up" for having a largely imperfect space. You might better spend some of the money elsewhere in that case (eg, mics, instruments, software, etc).