This is my personal opinion so you need to read it as such:-
When you pay capital away it is very difficult to build it up again as one always finds an excuse not to have the discipline of refunding the capital.
You have been paying off your bakkie since 2010 so you are pretty used to making this commitment every month, and your interest rate is below prime so you are alright there.
I would invest the R 70k in the stock exchange but maybe into a satrix type investment but not all of the funds in one hit unless you believe the market is going to go only in an upward direction( brave if you believe this as the market is very volatile), maybe what you need to do is pick 2 satrix investments and then enter the market and any dips below this figure you buy more (suggest initially R 10 k per fund).
You can address you bakkies debt differently as well - you could ramp up you monthly repayments to pay the debt off sooner, and once settled use this monthly commitment to enter the stock exchange but potentially buying shares as opposed to satrix type instruments. Whilst this is going on in the background, familiarise yourself with the mechanics of the stock exchange, attend free seminars, and courses if you can and broader your knowledge of our exchange.
There could be an issue with your installment credit in the sense that some institutions want you to renegotiate the deal if you want to change the monthly premium or pay off a balloon payment early but you would need to have a conversation with them to see whether they will waive the cost of changing/amending the terms and conditions. I am about to buy a new car and I have gained the institutions agreement that when I have spare cash I can address the early repayment of the balloon payment, or have the monthly premium adjusted downwards by introducing a cash payment or can address both items simultaneously should I so wish with no additional costs by redrawing the installment agreement.
As I said at the beginning this is my opinion, and, how I would address your situation, as I am a strong believer in not diverting capital to pay off debt especially where you have become acclimatised to monthly commitments