We have a small house (950 sf) that has only 1 cold air return- on the ceiling in the main living area. The duct work goes through the attic and down into the basement through a boxed in area in the corner of the kitchen that also houses the exhaust. We're starting a kitchen remodel and would like some of this space back... And it's never made sense to me that a CAR would be on the ceiling anyways.

I would like to drop the CAR to the lower part of the wall, directly under it's current location. We would rebuild the boxed in area to still house the exhaust, then use the found space for a pantry or the like....What concerns should we have and is this a job that requires a pro?

Your cold air return if you have heat only can be moved towards the floor. But if you have AC system the duct return should be placed near the ceiling. You can pan the wall framing and use it as a duct. Then simply place the air return grill onto the wall rather then keep it on the ceiling.

As far as using a pro to do the job. Not necessary for the actual job, but you need to properly size the duct or panning to be sure your not starving the air flow to the furnace or AC system if you have on.

As far as the exhaust pipe. Depending on the type of pipe that is used will depend on how much space you will require around it.
If based on what is normal for a gas appliance they used "B" pipe which is a two wall pipe. Code requrements for clearance is two inches away from the pipe on all sides. If its single wall pipe it is six inches. You also need to firestop the floor or ceiling within the attic where this pipe runs through. This must be done with metal.