I've tested it extensively, the only thing that makes a difference is luck. Charisma and end do nothing. When you buy a merc there is a roll for hp which usually varies a small percentage plus the luck bonus which seems to be 100 hp with 10 luck regardless of starting hp.
Well i just did a quick test and the results seem to say something else. I was buying HtH mercs in Redding. Those ninjas.
1. Build:
ST 10, PE 8, EN 4, CH 5, IN 5, AG 7, LK 1
Results: 128HP,128HP,132HP
2. Build:
ST 3, PE 6, EN 4, CH 5, IN 5, AG 7, LK 10
Results: 129HP,130HP,133HP