8.5 m2 sails normally require a 490 cm mast with a IMCS (Indexed Mast Check System) bend number of 28-32. (What you need to know here is that a 490 cm mast is stiffer than a
460 cm mast).
You can adjust the length of the mast to some degree using the mast extension, but you cannot alter the mast bend characteristics. Suffice to say you really cannot use an extended
460 mast in a sail designed for a 490 mast because the stiffness is different.
If you have a 460 mast, and it's a good one, you may be able to use it with more modern sails. (Tell me the brand of mast you have and any numbers near the base and we can probably tell if it's any good. If it's the pink (coral maybe?) or aqua (bluish green) color Fanatic mast it probably bends OK, but it will be quite heavy compared to a modern mast.
If you were to get an 8.5 m2 then you would want another sail in the 6.8-7.2 m2 range.
As far as fins go longer is not always better because the longer the fin, the higher the drag.
Huge fins have some advantages, but they require very wide boards to allow the sailor to control them.
For the Futura 141, the stock fin that comes with the board (maybe no fin if you get an 2010 board) is a 50 cm slalom design. This will work pretty well, but for an 8.5 m2 rig
you might want to go larger to about 56cm.
For a weed fin, you can go much shorter because weed fins have significantly more area.
There are several brands of weed fins here in the USA. I'd suggest a 39-42 cm weed fin for the Futura 141.
There is no downside to having larger fins in light winds. Larger fins generally get you planing earlier.
As far as your question regarding cambered sails, you do not need that much complexity at your level. Get something without cams (they add weight and complexity and will not give you any noticeable gains at your skill level) that's easy to rig correctly, get the light mast and hopefully a light boom and you should be good.
As far as cambers not rotating, that could easily be a rigging/tuning issue, but cambered sails are pretty much universally more difficult to get rotated than camless sails.
Hope this helps,