Gary there are lots of factors to anchoring, all of which become all the more important in depths over 70 feet and spring tides.
I had a quicksilver 640 and found a 7.5 kg bruce to be a good anchor size for the boat. Before Niggle says anything, I used to have a 7.5kg Bruce long story.
You need 6mm chain, depending on how you retrieve, through a gypsie on a boat of that size it would normally be 6mm. Two boat lenghts will be enough.
You might struggle to get more than 150mtrs of rope in your locker, if you use a bin then 200 mtrs.
Be careful over rocky ground because to much chain on a slack tide when you drop can pile up, then wrap around rocks and get caught, as I know only to well, slowly reversing can put an angle in the decent of the anchor and chain when dropping at slack tide.
You should always trip your anchor ie attach the top eye on the anchor with cable ties and use the eye nearer the business end to attach the chain with a shackle.
The reason for this is, if the anchor gets stuck in, the force applied by the boat will break the ties and lift the anchor out from the back end.
Quite often it's the chain that gets caught and that's a whole different ball game, if you remember the way you swung on anchor then try to reverse the process or more importantly in rocky areas, lift and re anchor on the change of the tides, especially Poole patch and Christchurch ledge.
When paying out rope, check the angle of the rope when the tension straightens the rope, in deep water the rope should be at a shallow angle, the more rope out of the water the better when anchoring in deep water, up to 5 times the depth. If the rope is at an acute angle you haven't payed out enough rope.
The cheapest way to buy 200mtrs of rope, 12mm 3 strand nylon is on line, they will also supply the anchor and chain and if you specify they will splice the chain to the rope.
Good luck
Mal.
Be careful many injuries occur whilst anchoring, it is one of the most dangerous activities on a fishing boat.