So here's what you need to do... measure your fireplace and get the largest insert that will fit in there. Remember this is a permanent installation and you are playing with fire. Cheapening out by buying a used insert is not necessarily the way to go.
You will then need to add to the cost a Stainless Steel liner with the same diameter of the flue of the insert (Typically 6") about $400 - $600. Do NOT cheapen out the installation by doing away with a liner. It will cause all sorts of drafting problems and creosote (potential chimney fires). Some will no doubt chime in and tell you their neighbour/brother/cousins etc.. did not need a liner. Dont pay attention. If the MFG puts it in the manual there is a good reason for it.
Finally, you will need a block off plate where your old cast iron damper was and possibly some insulation around the insert. If you need more help with this post your questions.
We make various brands of inserts for all budgets so ask away.