Tripod head recommendations please

I think about tripods as things that provide a steady platform, and don't really know why a tripod head would need a camera trigger. So i use nice firm and steady heads, that keep the camera rock solid.
But i don't do lots of follow-the-action photography (none at all, in fact), so i'm not one to comment. Sorry!

Palpas does not make the camera any quieter to a degree that would make it a deciding factor. (I have noticed that different models, even same models made at different times, produce a slightly different sound, but it never struck me that one was quieter than another).
It's effect on image contrast by reducing internal reflections is also not very big (if there is any at all - which i'd gladly believe there is, under certain circumstances. Can't say i have noticed any though), so i would also not count that as a deciding factor.

Palpas does crack, yes. But that's not a problem. If it ever would crumble a bit, it's easy enough to clean the debris out of the camera. No harm done.

So decide on the different features different models offer. The other major consideration is condition.
If to be used purely as a back up, another 500 C/M in good condition would do fine.
But getting a second body also is an opportunity to complement what you have with features you do not have. So myself, i would look at the larger mirrored 501 CM or 503 CW as a second camera, to be used alongside a 500 C/M. The 503 CW, or 503 CX for it's TTL flash.
But yes: pointless and a waste of money if you don't need or want those features.

Thanks QG - appreciate it. I am thinking of the trigger purely because I try and control the 'space' but not the pose - so they move around a bit ;-) I should also say I am just a beginner, so I probably have no idea as well! But that's the theory behind it for me anyway.

Being a beginner (not something to be apologetic about) doesn't mean your thinking isn't up to scratch. And (without being patronizing) i certainly think your thinking is good.
It's just not something i do, move the camera about while shooting with the thing on a tripod. Which just disqualifies me, as not suited to comment on the tripod head thing. Just wanted to let you know that, instead of just (bluntly?) ignoring the tripod head part of your question. ;-)

I know this is not real sophisticated, but I saw one of those, and they struck me as being more for video. There was something about that pistol grip bit that just seemed like it would be unwieldy when it was bearing some camera equipment.

It seemed to me like it would like a ball and pan head whose positioning would be complicated by leverage. I don't know this certainly, because I have never used one; but, I took one look at that design and went back to looking at more traditional equipment.

I use the Manfrotto 3265 ball grip. I am comfortable with it when leveling or pointing the Hasselblad camera and a short lens. I think the 322 head is a better idea; it will be more stable because the camera sits closer to the ball and safer when using a longer lens. But, the side handle may interfere with your hand when winding. So, moving the handle to the left of the camera will be better.
All my Hasselblads bodies have cracks in the Palpas; no other problems. They will work great, Palpas or Not.

I have one of those heads, I used it with a Nikon D200 but after a while found its operation annoying and went for a decent ball head (an Acratech).
While that's a subjective comment I second m allegretta's commment about it interfering - it's a right-handed device and the handle will protrude directly underneath your 500CM in the path of your winding.

Manfrotto's pistol grip heads have never given me the impression of being particularly stable. I use an Arca Swiss Z1 ball head, it's great; moves smoothly, head doesn't flop, holds heavier equipment well, good tension adjustment.

I had one years ago. It would probably be OK for a Hasselblad but useless when tilted for anything with a bit of weight. With a little practice, I think that an Arca or RRS ballhead would be good as both have friction adjustments and only need a quick twist to lock them. With a cable release, and a steady hand you could probably get good results without locking the ball at all. The camera with a 150 isn't very heavy.
Tripod heads aren't things that you change often, It took me a while before I bit the bullet and bought an Arca B1 but I have never regretted it. I probably have an Arca plate or 2 for the V series hanging around that you could have if you go that way.