Mast. I'd be looking for an agent in France for Nedal, who can make conical tapered aluminium tubes up to the size you'll need. I used one of their 220mm diameter tubes as the mast on Tystie, carrying a sail of 54 sq m, but the AR was low, and the overall length was only 12m. However, I think you'll be OK with this diameter and an overall length of 13m carrying a 50 sq m sail.
Battens. Carbon fibre will give you the lightest, strongest and stiffest battens, but at a price. Just comparing GRP and CFRP available from the source I use, https://www.carbonfibreprofiles.com/, carbon is three times as expensive. It depends how deep your pockets are. The larger the sail, the more desirable it is to reduce weight, but GRP is acceptable. I'd choose 50mm OD x 46mm ID for a 5.5m batten in this size of sail.
Cloth. When I've experienced breakdown of the sailcloth, after many ocean miles, it's been near the leech of the lowest panels. This is where there is some fluttering and flapping that doesn't occur where there is more tension in the sail, higher up. To counter this, I add weight in this region, not subtract it by using lighter cloth. In a 50 sq m sail that is intended for long term cruising, I would not use less than 8oz/sq yd - Weathermax 80 or Clipper Canvas 7.2 US measure (= 9oz/sq yd) (still available, I think direct from Heathcoat, but if not, Weathermax 80 is right for this sail).
Sheet. I would be happy with the sheet blocks being just forward of the steering pedestal, so long as the battens are no longer than 5.5m, and so long as the top sheeted batten is extended aft of the leech (it's the top span that tends to get caught behind the upper battens, the lower spans are better behaved). Putting the sheet blocks on the pulpit would have the sheet sweeping dangerously overhead as you stand at the wheel.