Phonics are it for me as well, I just love my bop kit(12,14,18 in rosewood). I think they tune up nicely and if you're going towards the higher end of tuning then the phonics will suit you well.
As Magnus said, the 20's are kind of harder to find, both 18 and 20 will cost you a bit. The last 18"(last week) sold on ebay for $799 and I don't think it had the single tom post. It was a very nice original wrap drum, 20's go for around 400-500 or so.
When I started out on my quest for a rosewood kit I was only going to do a 12", 14(mounted tom) and an 18" floor tom to bass conversion. That might be an option. If you do go that route I can help walk you through it. The problem you encounter with sonor 18" floors is that they are drilled for 9 lugs and they are about a half an inch closer to the bearing edge so you really need to get shorter claws and t-rods. It's much easier to use the tom with just a riser and play that way. When I did my conversion(18x16) I was very happy with it as a bass, it had a little more volume than a factory bass(18x14). Since then I found a rosewood 18 and put my floor tom back on my rock kit.
Good luck on your search, Gary.