I would consider a 5 gallon bucket a practical minimum for zucchini. They become pretty large plants and can use water quickly so a larger volume container would spare you from watering as frequently. You could also get a 15 gallon container and put more than 1 in it, but I would still recommend sticking to no more than 1 per 5 gallons of mix to avoid their becoming water stressed if you find mid season they need water more frequently than you can provide.

In terms of potting mix I really don't know what you have available in SA. I would say get a mix that is coarse and drains excess water quickly. If you can find something where the mean particle size is around 1/8th inch or so that would be ideal. Mixes based upon sphaghum peat poss (not other peat types) are fine, but decompose fairly rapidly so you will probably want to replace it every season or so. Mixes based on pine/fir bark fines (the particle size will range from super tiny to a max 1/2" sized pieces with the bulk being around 1/4" or so. Coir, if available would also be OK, but again a fairly short life span due to the rate it decomposes. In all cases adding perlite in a medium/horticultural grade would be advisable to help keep the soil from compacting too quickly.

You can learn more about what makes a good container soil here.

In terms of 'plant supplements' I assume you are asking about plant nutrients? You will do best with a water soluble fertilizer containing all plant nutrients, but I don't know what you have available in your area. Do you have products like Miracle Grow? If so their general purpose fertilizer would be a good choice. It's likely going to be labeled as 24-8-16. You can learn more about providing good nutrition to your container plants here.

Last, but certainly not least, are you aware of how much zucchini a single, healthy plant can produce? There are half joking horror stories about people losing friends and neighbors locking their doors to stop the zucchini grower with just a few plants from dropping off bag after bag of extras ;)

Hi, thanks for this, it's going to help when I go to the nursery and start talking to the people there.

I'm reasonably certain that the nursery I'm thinking of going to will have pretty much everything you're talking about since it's geared towards serious gardeners.

I'm not sure about Miracle Grow, I think that's an American brand, but I think there are similar products here.

I must say fertilisers have always been a bit of a mystery to me - different plants needing different ratios of whatever goes into them and so on. I think I killed my disa orchid by giving it the wrong kind of fertiliser.

I've heard that zucchini are incredibly prolific, which was part of the reason I chose to start with that. I love zucchini, so am planning on eating the bulk of the harvest. :)