welcome to the madness :-)
bimini top!!!! in Scotland!?!
Your choices for this are quite limited, you can get some on ebay but i have no idea on how useful they are, someone else may be able to answer that more specifically. i'd be inclined to suggest just use the correct clothes for whatever the weather is.
I assume you mean an A frame for the back of the boat, you would be best putting a wanted advert on here or keep an eye on ebay, you should be able to pick up a single tube quite easily i would have thought if you have a bit of DIY skills to make it fit. some options here perhaps- http://www.ebay.co.uk/bhp/rib-a-frame
I am not sure about the prop question but my immediate thought was a 4m boat with 30hp 4 stroke isn't going to pull a skier easily. I think those boats are rated for 50hp, it should be on the transom plate. perhaps the reason yours only has a 30 is they were known to have problems with transoms cracking and the owner/builder decided to use a lighter engine.....my speculation.....or they could have just wanted to save the money, either way 30hp is going to be enough to have fun with on it.
given what you are asking now it may be worth just using it for what it is and get the boat you really want rather than plough wedge into this one?