Portraits with a speed, are best done handheld if possible. That said, you want to be able to use the rangefinder. Presuming you have a pacemaker speed with a kalart, that can work with 90-210mm lenses. So, I'd look for something in that range. The speed with a 300mm and bellows racked way out is a bit unwieldy, handheld. (if not doing handheld, then just get a nice monorail camera and use on a tripod - it's much more versatile, ultimately.)

The aero ektar is really the best choice if you can find one for reasonable money. I've found them for $325-375 before, which is only a bit over your range. Another good choice is the Pentac 8" f/2.9 lens. Lighter and usually cheaper than the AE, it will also mount up to the speed nicely and work with the RF. There's also other barrel lenses in that range, aerial lenses or otherwise. Even a good 150mm+ enlarger lens will work fine.

-Ed