I think about buying a scart to USB converter for recording my old VHS tapes.
Will kdenlive do the trick and which converters are supported?
I run Debian wheezy with KDE

Kdenlive pretty much relies on other packages to capture video. Eg it uses dvgrab http://linux.die.net/man/1/dvgrab to capture video from firewire (which is what I use)

It has a FFmpeg mode of capture which I think might be using the Video4Linux2 package. Your USB convertor may be able capture video via this method. But I could not be sure.

The question you need to figure out is - can linux capture video from your USB convertor at all. Keep Kdenlive out of the picture and research if your convertor can work under linux.

PS. I just noticed on the DVgrab page
"dvgrab also supports UVC (USB Video Class) compliant DV devices using Linux kernel module uvcvideo, which is a V4L2 driver."

So maybe if your capture device is USB Video Class compliant you will be OK.