Hi,
Before your question can be answered fully, some more information would be useful in regard to which specific saildrive model is fitted to your boat, and what type of propeller (folding or fixed) that you have fitted, and if it is a folding prop, which manufacturer and specific model.
A Zinc anode, as you have noted in your posting is correct for use in sea water, and unless you sail quite a lot further up the Severn estuary, you are unlikely to need an anode more suited for brackish water. I've never noticed any difference in performance of anodes between different manufacturers, and so probably the most important thing is to ensure that the new anode you buy is the correct size for your particular sail drive. So you need to determine which model of saildrive is fitted on your boat. Is your boat fitted with a folding propeller, and if so you will need the specific anodes for the type of prop fitted.
In regard to antifouling paint, and now that the EU have decreed that antifouling paints should be almost as useless as a chocolate fire guard, I'd suggest that you buy the cheapest possible, but have a chat with yachties at your local sailing club to ask them what they use and which if any they find gives them good protection. A standard 2.5 litre can of antifouling should be more than enough for a single coat. In the past I have used International Cruiser Uno, and this does spread much further than some of the lesser priced paints which helps to make it a little more competetive in regard to cost.
FOR THE SAILDRIVE ENSURE THAT THE ANTIFOULING PAINT YOU USE IS SECIFICALLY SUITED FOR USE ON ALUMINIUM, and do not paint over the anodes.
Most antifouling paint is, or has been copper based, and if you make the mistake of using a copper based type on your saildrive it will over time eat your saildrive away. Do not therefore be tempted to "save money," or listen to anyone who says it doesn't matter what kind of antifouling you use on your sail drive unless you are completely happy with the thought of getting towed in and having to replace the drive after the old one has disintegrated. The same aluminium friendly antifouling used on your saildrive should also be used on the aluminium ring around your rudder shaft where it exits the hull.
While your boat is on shore is the ideal time to change the oil in your saildrive. For this you will need probably the biggest flat bladed screwdriver you can buy. Do not be tempted to use one that you have already got in your toolkit if the blade is in any way worn. The plug is located at the very bottom of the saildrive, it is made of marine grade stainless steel which is quite a soft metal, and where the slot cut into it is easily damaged by worn screwdrivers. You will also need a translucent container such as an empty plastic milk bottle to catch the old oil. This you should keep for at least a couple of days to check that there is no water within the oil, after that you can then dispose of it into your boatyards old oil collection point. Water in the oil would mean having to get the seals changed, a boatyard job unless you are very mechanically competent.
You mentioned that your boat is a 2006 model, so if it hasn't been done, then you should be thinking about whether to change the saildrive hull seal. Volvo state that this has a life of around seven years, and anecdotal comment suggests that figure is very conservative with reports of them lasting many years longer. Mine was changed soon after I bought the boat, at which time it was nine years old and looking like it was almost brand new. Therefore I don't think that it is critical for it to be changed at this time, but you have to weigh that up what your insurer might say if it leaked and you put in a claim. My thoughts are that their reply would be along the lines of " tough $&!¥ matey, get lost."
Hope it all goes well.
