The only problem with going with a full set of poly bushings is though it stiffens the car a lot, it can make the interior a little rattlely because of the transmitted vibrations.
If you have under dash/interior rattles now, you will REALLY have them after going to a full poly set up. If you are going to open road race the car, I'm right there with you going to a full poly set up... but...
just replacing the missing lowers will provide an improvement you can feel for about 30 minutes worth of work (If your car isn't from the rust belt).