Hi,
The rangefinder on the Ikontas - the models that came with one anyway - is uncoupled. That means that even though the rangefinder will tell you how far away the subject is you will still have to turn the focus dial on the lens until the numeral matches up to the same one you got from the rangefinder. Otherwise, you just 'guesstimate' the distance and set the focus that way, which is generally how I use my Ikonta Super B. I think I only used the rangefinder once just to check if it seemed to be in proper adjustment.
Guessing the focus distance takes a little practice, but as you get a feel for how far away things are that you might be photographing, you'll be pleasantly surprised by how many sharp negatives you are rewarded with - even with larger apertures. So I don't know if an accessory rangefinder will be useful to you. If you're willing to waste a couple of rolls of film exploring the mysteries of guesstimate focusing I think you just might discover you don't need one.
Have fun with your new folder, but watch out! They're addictive!
Joe