I would stay away from the multi blade heads HK stocks as most are seconds of rejected by the original distributor whoever it may be.
There have been many posted threads on other forums where people have ordered and received heads from HK that the head block its self was misdrilled and could not even be used , at this point it is just a paper weight.
The CopterX head is of good quality and myself I have 2 of their 4-blade 450 heads and have not had any issues with eather one. However before you install the head on your heli I recommend you disassemble it and reassemble it to insure all bolts have thread locker and that all bearing are free (smooth) and clean.
The SE v2 model is a much better choice for scale if you intend to go with a 5-blade head in a MD 500 because it has a high tail rotor ratio than your Pro model which you will need running the 5-blade head.
HK does carry the Tarot ZYX 3 gyro FBL unit which for the price is a fair unit to use on a 450 project of this type and there are many threads on other forums such as RCGroups.com that has quite a lot of good information on this unit.
Here is another good quaity 5-blade head you might look into more , it is not as cheap as the CopterX head but there again in this hobby you do get what you pay for.http://www.cnchelicopter.com/servlet/the-2077/xmhobbies-trex-450-etc/Detail