Those are the old-style cylindrical keyed locks U-locks, they haven't made those in years.. but yeah- I'd avoid buy old used U-locks.
New locks have a redesigned straight flat key system. Just be sure to use a lubricant (chain oil or such, not WD-40) to keep the lock from rusting and seizing and breaking your key off!
Can you break this? I use it on my MB.
Amazon.com: Kryptonite New York Fahgettaboutit 1410 Chain Bicycle Lock with New York Disc 3 Lock Chain Bicycle Lock: Sports & Outdoors
I couldn't break the U-lock (it's a U-lock and a high-grade steel chain?). I could probably bust that chain off (chains are vunerable to prying and cutters- they have seams). I hope you're locking the U-lock to your frame to whatever you're locking it to and not locking the chain around your frame only- just to be sure. It also goes without saying that your locking is only as good as what you're locking it to..chose metal (thicker the better- in full public view Vs. smallish evergreen shrubbery behind the mall?
Also, you don't need to buy a brand name Kryptonite U-lock..they're $10-15 more and it's just a brand name! I have a Bell that I got for $14 and it's been used for years..