The ASUS with upgraded RAM and an SSD should do you fine. AMD is not my favourite but realistically they're not bad CPU's. Usually slower in benchmarks but in the real world those mean little. I will say that AMD chips tend to be less energy efficient compared to Intel which leads to less battery life, but most people keep laptops plugged in whenever possible anyway.
If you only have the money for one upgrade I highly recommending prioritising an SSD over RAM. You notice a much more significant performance enhancement from the I/O speeds of an SSD than you will from adding more RAM. The 5400rpm HDD's they put in laptops are especially terrible. But even a budget SSD will be so much faster, you will seriously notice it constantly, just make sure you get one from a reputable company. I have SSD's from Crucial, SanDisk, and Kingston with the former being cheapest. All are good, zero problems with any SSD I've ever had.
Not sure about SA prices but Amazon UK has 240GB SSD's for as low as £25 (~$30 USD). If you need 500GB it's around £50, or for 1TB it's £100.