HDD not manufactured any more

I've just recieved my ReadyNAS Ultra RNDU4000 and i've installed my 2x 2TB Samsung HD204UI drives, it's syncing up as we speak. Now Samsung has joined up with Seagate so their drives are no longer for sale at most places here in Norway, some stores still have stock, but at much higher prices than when i purchased the drives at the start of 2011 (Higher prices due to the flooding in Thailand - Poor people!).

I don't need more space right now, but my requirements will increase as time goes on and by that time i won't be able to purchase the exact type of disk being used today. Is it worthwhile to just buy 2 more disks now at the higher price to avoid complications in the future or can i just buy 1 or 2 more 2TB drives from anoher manufacturer, Seagate for example, and add them as i need?

If mixing manufacturers would cause reliability problems i will just buy now, if it's a non issue i will wait and buy in more disks as i need.

Thank you

Jack.