Liz, to be honest the one thing about photography is that you can just keep on spending and still have a wish list on top of that! Make the move over to a DSLR with whatever lens, get used to it and then review what else you might need. You talk about IS as an essential, but if you are photographing close up, you should be doing so on a tripod anyway, in which case the IS will be superfluous.
It is a long time since I used a converter and always cursed it for the loss of light and clarity, (although it wasnt that expensive) on an old film based system. Whilst I might find use for one now, I tend to have a range of lenses that generally cover my needs so steer away from such 'compromises'. What am I saying? Wait and see whether youbreally need one, once you have your new camera and lens. Magazines and even forums will provide youbwith such a plethora of options you can become fazed and convinced that you MUST have some bit of equipment. Thats fine if you have pots of money or can justify it, but when starting out with a system, it is far better to take your time. HTH.