It's all a bit of a compromise. I suppose the first question I
should be asking is how much line do you need to catch a reasonable
size marlin? As we'll be stand-up fishing, anything more that 80lb
line would be pointless. A 50W reel can fit 550yds of 80lb mono.
Is 550yds enough? We'll be chasing them down with the boat, but as
I assume marlin can swim at least as quick as sailfish and obviously
pull a lot harder, I reckon 550 sounds a bit light on.

I could go 50lb, but not only would I be reducing my chances of
landing one, I'm not happy with the thought of killing one by
fighting it for too long.

That gives two options - use a bigger reel or use a thinner line.
The thought of stand-up fishing with a 3kg+ winch doesn't hold much
appeal, so I'd prefer to limit the reel size to a 50W.

I've had plenty of experience with both braid & dacron, but no
experience with marlin. I use dacron for sharks, macks & tuna, and
when bottom bouncing I use braid (stick a bit of sticking plaster on
my thumb). In both cases I use a bimini double attached to a swivel
with a cats paw). I then just clip on whatever leader is
appropriate for what I'm chasing. The sailfish I've caught have
been on mono though.

I've just bought a 24-37kg stick with roller guides, so shouldn't be
any problem winding a join on. My dilemma is what kind of join in
what kind of line.

Rob