RE: Moving AR transactions form one company to another
How much history is in the AR database, and is the AR database also a GP database?
There are a number of things you can do, but I think your best bet is leaving the history in the separate AR database and running reports on the historical data as necessary. If you have someone on staff who knows SQL, you could create SQL view that holds data from both companies. Check GP Table Reference to see which tables you'd need to include in those views. You may also need to have that person build some logic into the views that handles exchange rates. Again, I think leaving the data in the old database is your best bet.
Getting the actual data out of the other database and into your new database might prove challenging. You could use integration manager to bring over AR transactions and SOP invoices. You'd then need to post them in GP, review and make sure everything ticks and ties to what's in the old database. Lastly you'd need to import cash receipts and apply/post. This could also be done manually. I'd recommend testing the heck out of any integration manager jobs too. I've never used IM for historical data.
You could also have your dba build an SSIS package that moves the data from your old database to your new database. The risk here is that there's no GP validation like there is using IM. You could also do direct SQL insert. What you'd need to do is validate that you're not importing duplicate transaction numbers into your new database. Or you could just append an "_OLD" to each transaction number in your old data. Again, test the heck out of this one too.