The health check does have a fee if you're not on a Discovery health plan - R250 this year. The fitness test costs R375. You should easily make Gold if you do both. If you buy yourself a cheap step counter (Black Friday coming up... there should be loads of deals) and do 10 000 steps per day most days, I'd say you don't need the fitness test, since you can earn enough fitness points through the step counter to help you reach Gold.
I'm not sure you have to have another product - this page
https://www.discovery.co.za/vitality/join-today seems to suggest you can just join Vitality?
In terms of what you can save, I'm single, and a really low spender (only spend on the credit card 6 months of the year), and I've earned R1 300 in healthyfood cashback this year. So this would be double if I were buying food the whole year - R2 600, and Vitality cost me R2 628 this year.
If I were you, though, I'd consider the credit card. The monthly fees are the same as most other cards, and if you opt in to Discovery Miles (R420 pa, I think) and get to a decent Vitality status, you can earn some decent cashback via the Miles Multiplier.
I'm on Diamond status, and I've earned 16 365 Discovery Miles this year, which equates to R1 636 worth of spending at various retailers - and the list where you can spend Miles is growing (I don't spend them - I convert them to Avios and use them for flight upgrades). Again - I'm single and a low spender on my card, so this would easily be more for a "normal" spender.
I've saved R821.10 on a 9-day car rental for a holiday in Cape Town, plus R886 on a kulula flight.
I do exercise regularly, making my Active Rewards goals every week, so I've also saved essentially R1 650 on my running shoes, and will get another R1 711 back via the flight booster for my kulula flight (though perhaps I should offset the cost of my fitness tracker against this, since I had to buy the fitness tracker to enable me to make my weekly goal - but that was just a one-off purchase and I can get these discounts each year). If you're active already, you could get a decent fitness tracker with HR monitor and be eligible for these discounts as well via Active Rewards.
So in my opinion, it's totally worth it.