How many clients (minimum) will justify using SagePay or Netcash
That depends on how much you earn per client per month on average. Netcash charges a monthly service fee of ~R200 (plus a nominal fee per transaction), so a single client might justify using it.
I switched from EFT customer payments to debit orders and credit card payments several years ago and have never looked back. There will always be some customers who refuse to pay by any means other than EFT, but being able to predict your cash flow with almost 100% accuracy and go to bed at night without worrying about when your customers will pay you is priceless.
Initially I used my bank's debit order service. BIG MISTAKE. Their fees got exorbitant very quickly. I was reluctant to use Netcash initially because they only pay 90% of your debit order upon transaction and the other 10% after 30 days, but I got used to it pretty quickly.
Before switching to Netcash I took a close look at comparable offerings, and Netcash was the clear standout. I read some horror stories posted by guys who had used less reputable providers.