You can get one for about $300 or you can get aftermarket for about $240. Since you end up with a tactical clone in the end there is no since in paying a premium for a HK barrel when the Jarvis barrel is just as good. However, you still will not have a tactical just by putting in the barrel. You still need the match sights and trigger and still you have a clone. IMO just buy a Tactical so you will have the real deal. Piecing one together is penny wise but pound foolish. When all said and done you end up paying a lot more for your clone than you would have spent if you had just bought a real Tactical.
Tactical, NIB complete: $850-$899
USP:$700
Tac barrel: $300 (HK) or $240 (aftermarket)
Match trigger and install: $150
Match sights: $135
Tactical case: $75
Cleaning kit: $25
Total: $685 just for the add ons, not counting the $700 for the pistol.