I am anxiously awaiting to read the responses you get to your question because I am in the exact same boat with my 4 AND 2 year old daughters. They are both in my bed each night and it's getting a little crowded. Funny thing is that they don't like to sleep together in my 4 year olds double bed. I too struggle with the idea of destroying their security of sleeping with mom and dad- my 4 year old is just really afraid of the dark and my 2 year old has a lot of nightmares. So I really have no good advice, but did have a couple of thoughts based on what you said in your question. Is there any reason you couldn't get the twin bed now and just have the younger one move into it when she's ready? Also, are you sure a twin bed is big enough for both if your plan is to have them sleep together? I ask this because I always planned on my girls sleeping together so bought my oldest the double bed and my youngest just can't stand sleeping with her sister. One likes the dark, one doesn't. One likes to snuggle, the other doesn't. So while I was hoping that being able to sleep with her sister would help my oldest stay in her bed, that hasn't been the case.
Also, you can get mattress pads for toddler beds that are water proof but not plastic. I have 3 of them because I too thought the plastic sound was making it hard for my older one to sleep. I bought them either at babies r us or target. If you can't find a non-plastic mattress pad, look at target for a waterproof pad. I have one on my king bed and my daughters double bed for just in case purposes. It goes under our mattress pads, but on a toddler bed it would probably cover the whole top of the bed and you could just put the fitted sheet on top. That saves the mattress from wet diapers without being plastic. That might help. Good luck, and I'll be checking out the other responses for some advice.