If the data structure is the same ( or very close ) for the old data and the new data, then you may be able to use Goya's refreshFM product. It is intended to merge data. The ususal intent is to move current data into a new version, but should work just as well pulling in old data. ( I think ) Check out the web site http://www.goya.com.au/refreshfm
If you have questions ask them, I have found that Nick has been very helpful when I had questions about BaseElements.
The other method is to manually import the data.
One thing to check no matter what method you use is to make sure the keys don't overlap. If when you purged the data, you reset the auto-enter value for the primary keys for your data, the old/purged data and the curren data will have the same key values. This will pose a serious problem. There may be ways around this, but that could involve a lot more work then just importing data.
Of course the easiest, is to just provide the old file and tell the buyer that old data is in this file and the new data in that file. Your idea is much more attractive/easier for the buyer of your company. I am assuming that you were concerned about peformace with all of the old data in the file, hence the purge. The new owner may end up with the same concerne down the road.
Best of luck with the sale of your company.