I bought a Lowrance LMS-320 from the states 3yrs ago and I made a huge saving. It will work over here, but you obviously have to buy the correct Navionics chart for your area separately.
As you've said you won't get a warranty, but I've never needed mine and I've given mine a fair amount of abuse over the years and it still workd perfectly!! Whether its worth the risk is entirely up to you, but Lowrance units tend to be pretty much bombproof. (touch wood!!)
If In Doubt, Go Flat Out!!