Pick it up - you can buy a new recoil spring assembly for $50 or so (highly, highly recommend that be your first purchase unless the seller can document when the assembly was last replaced, round count on it, etc).
Maybe buy some mag springs as well; on the 45 those are a known weak/wear point...USP45 mag springs go weak quicker than the 40 or 9mm springs do for some reason.
Check the sear spring; they can crack/break. If it's solid and in one piece, you should be good to go.
Check the HK Web Shop for part prices, and if you don't see what you need, call them. They stock a lot that isn't on their web page. Odds are you can buy it from HK directly for less than you can from other vendors online.
If you pick up a factory threaded barrel, you should also pick up the factory USP Tactical recoil spring assembly - yes, there is a different one that they use on guns that might be shot suppressed. There's enough differences to make it worth buying if you plan on shooting it with a can. Costs the same as a normal assembly and you can use it in the place of a normal assembly too...so if you know you're going to shoot it suppressed a fair bit, just buy a new USP Tac recoil assembly instead of the standard assembly.