If you updated to the current version you would have the option of archiving rather than purging which would clean up the clutter in the active file but keep the closed orders available for reference.  That said, there really isn't much risk with purging the old closed sales orders because all the important information relating to what was actually shipped is in the Invoice files and purging or archiving sales orders does not touch the invoice files.  As for how far back to keep, everybody is different.  It depends on how much you are likely to go back and look at a closed order and if you do that a lot, can you retrain yourself to use SO-H that looks at the invoice(s) that came from the order rather than SO-A/SO-T to look at the order itself.

If you are printing Acknlowledgements one at a time or a range of consecutive orders together and specifying the Sales Order number(s) then it is using the index and the number of records in the file really shouldn't matter.   Reindexing BKARINV might help.
 
Purging or archiving sales and work orders will speed up the Rebuild Stock Status that occurs in IN-A and is an option on a number of reports.