OP: I think you're a little confused about state MG registries (probably due to AlexanderA's comment). VA is a bit of an oddball regarding machine guns in that they have a state machine gun registry separate from the NFA. Some other states have various quirks like this, but by and large this is NOT the norm.
Don't want to state the obvious, but if you're not in VA, you don't have to worry about their laws.
My understanding is that PA does not have a separate state registry and that they default to the NFA rules, which means your registered receiver/trigger pack/sear would be the only machine gun as far as anyone is concerned. You would not have to register the host - it would be similar to owning a suppressor where the can is registered, but you can swap it between guns (hosts) freey without any registration necessary.
This is why it's important to list your state either in the OP or in your profile. Otherwise it's very easy to get entirely correct advice that is not relevant to your situation.
HK sears/trigger packs are usually the way to go because of the flexibility in which host you want to use (and their price typically reflects this). One advantage of going the trigger pack route is that you should be able to replace the internal parts in the case of wear/breakage. If you have a registered sear and it breaks, you're generally out of luck unless you can find a gunsmith with the ability to fix it. On that note, it may be a good idea to have insurance on your gun. I don't know about you, but if I broke a $30k gun I'd be very happy to get at least something back...
Also, not to open up another can of worms, but with trigger packs/sears, there is also the issue of timing. I'm not sure if HK Sears need to be timed to the trigger pack or timed to the gun, but it is something to think about when switching around between guns.
