DD was born in June, and she's a warm person anyway. For 2 weeks (when it was rather warm), she pretty much wore nothing except a nappy, sometimes a vest, and would scream otherwise. All the tiny clothes were barely worn. 0-3 months, however, much more useful. A PP said about generally one layer more than you is right, but I tend to go on one layer less with DD as she's always so much warmer than I am!
I'm not sure 20 vests is too many. Sometimes you'll use one a day, and other times it could be 3, 4 or 5! I'd probably go for 10 babygros for a May baby, assuming that you'll receive more as gifts. Again, sometimes you'll use one, sometimes more. In DD's case, it wasn't really sick that was the problem, it was explosive up the back shits! A couple of cardigans, for if it does get a bit nippy.
And YY to rompers, bit cooler than a babygro if it's warm and they fit longer because their feet stick out!
You can always buy more of something if you need it, but be wary of online shopping during night feeds 