plate chillers are really hard to clean because you cant get inside most of them. they have lots of little nooks where stuff can get trapped. if you can guarantee that it was only used with some type of glycol, it might be ok to use after a really good flushing. if it was or could have possibly been used with any other chemicals, i wouldnt risk it.
i would hook that up to my pump and boil kettle and circulate 10 gallons of boiling water thru both sides for 10-20 minutes. then hook it up to a garden hose and flush clean water thru it for another few minutes. if its stainless brazed with copper, use some BLC line cleaner or other alkaline soloution to break up anything else inside. unfortunately copper and acid dont mix, so you cant use anything stronger without damaging it.