Hiya
You're definitely right to delay a booster for as long as possible (or at least till your DD is four)!
There's more info on harnessing children 18kg++ on this page
http://www.childrestraintsafety.com/extended-harnessing.html though I do suggest you read the Britax Bulletin on car seats here:
http://www.childrestraintsafety.com/downloads/britaxbulletin.pdfOther then the information in the above two links, there's not much else I can tell you to be honest. Though, there is a seat coming in to the country some time this year called the Compass True Fit. It rear-faces to 15.9kg but forward-faces to 29.5kg. I've been fortunate enough to have a little play with it and it's really quite nice and very easy to adjust the harness height. It might be something to look at, depending on how quickly and how heavy your DD gets!