can I replace my existing sledge door handle with this one with a key lock and still use the interior door handle with this one

No. And I assume you are referring to a Schlage (not sledge) passage or lockset. There is connecting hardware between the frontside and backside of the lockset and there must be actuating hardware on the inside handle to lock/unlock the door from the inside. A passage lock to which you are referring must be totally replaced with a keyed lockset. If you are using Lowes as a source for your materials, you will probably find that you will have to order on line the aged bronze keyed lockset as many Lowes do not carry this in the store, even though there is a demo of it in their hardware department. Time to order is only two or three days.

The Schalge F51ACC 716 keyed door handle shown will work with a 2 1/8" mounting bore hole. It is a complete lock with interior and exterior levers.