Does this build have value and performance?

Yes. That's a very reasonable build.

If you don't mind the video card being gimped by ryzen, then it is fine. To date the ryzen's are really only good up to GTX1060s. If you want to get to that last 5-10% from the GPU you'll need to go Intel.

What exact power supply is that?

Get a X470 motherboard, otherwise your new Ryzen wont boost as much / not as much performance.
The X470 SLI is not as cheap but for the price it's a good value.

While X470 chipset MoBo could offer a tiny bit better performance over B350 chipset MoBo, the performance wouldn't be the main thing why to go with X470 chipset MoBo.

What would be main thing is that B350 chipset MoBo needs to have BIOS that will support 2nd gen Ryzen CPUs. If it doesn't have BIOS that supports 2nd gen Ryzen chips, you'll need to update the BIOS. And to do that, you'll need 1st gen Ryzen chip.

For MSI B350 Tomahawk MoBo, it's BIOS must be version 7A34v1C with release date of 2018-01-29 or newer BIOS. If it has older BIOS than that, your system won't POST.

R5 2400G has even worse value than R5 2600 by being in the 35th spot. Also, do note that while R5 2400G does have on-board graphics, it's not a CPU but instead an APU. And as far as CPU side of the performance of APUs go, they are even worse since good part of the APU die is used up by the on-board graphics.

In R5 2400G, there's Vega 11 GPU which has so poor performance that using it for gaming is painful. Even the GTX 1050 Ti, which can do medium settings @ 1080p is a lot better than Vega 11,
comparison: http://gpu.userbenchmark.com/Compare/AMD-RX-Vega-11-Ryzen-iGPU-vs-Nvidia-GTX-1050-Ti/m401440vs3649

$800 AUD can get you this Intel combo, with money to spare:

PCPartPicker part list / Price breakdown by merchant

CPU: Intel - Core i5-8400 2.8GHz 6-Core Processor ($248.00 @ Shopping Express)
Motherboard: MSI - B360M PRO-VDH Micro ATX LGA1151 Motherboard ($124.74 @ AX86 Gaming Systems)
Memory: Corsair - Vengeance LPX 16GB (2 x 8GB) DDR4-2666 Memory ($269.00 @ Shopping Express)
Total: $641.74
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-05-22 11:46 AEST+1000

Few words
No matter how you look at it, i5-8400 is better CPU than R5 2400G APU,
comparison: http://cpu.userbenchmark.com/Compare/AMD-Ryzen-5-2400G-vs-Intel-Core-i5-8400/m433194vs3939

Downside of non-K Core i5 is that you can't OC it.
While the Ryzen CPUs do have OC ability compared to the non-K Intel CPUs, majority of users don't OC their CPUs. CPU OC is more for an enthusiast thing than common practice. Still, there are gains with CPU OC to increase minimum FPS and reduce stutter but that doesn't apply to all games. Some games (e.g CPU bound) do benefit for CPU OC but other games (e.g GPU bound) doesn't benefit almost at all from CPU OC. And of course, there are also downsides. Running CPU clock on constant high clocks (e.g 3.8 Ghz) will wear out CPU much faster than running it with stock clocks (e.g 3.2 Ghz). Not to mention the additional heat produced by the CPU which in turn increases the noise CPU cooler makes to cool it down.

But if you want to have CPU OC ability with Intel build, you need to go with K-series CPU and Z-series MoBo, e.g this one:

PCPartPicker part list / Price breakdown by merchant

CPU: Intel - Core i3-8350K 4GHz Quad-Core Processor ($229.00 @ Mighty Ape)
CPU Cooler: be quiet! - Dark Rock 3 67.8 CFM Fluid Dynamic Bearing CPU Cooler ($89.00 @ PCCaseGear)
Motherboard: MSI - Z370-A PRO ATX LGA1151 Motherboard ($159.00 @ Umart)
Memory: Corsair - Vengeance LPX 16GB (2 x 8GB) DDR4-3000 Memory ($272.00 @ Shopping Express)
Total: $749.00
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-05-22 12:19 AEST+1000

Few words
Due to the budget limitations and necessity for aftermarket CPU cooler, couldn't put in i5-8600K. But did manage to put in i3-8350K, which by value is 3rd best CPU there is. Only i5-8600K and Xeon E5520 have better value than i3-8350K. Best part is that i3-8350K is also a lot better than R5 2400G,
comparison: http://cpu.userbenchmark.com/Compare/AMD-Ryzen-5-2400G-vs-Intel-Core-i3-8350K/m433194vs3935

Also, there's little difference between i3-8350K and i5-8400 where Core i5 has 6 cores/threads compared to the 4 cores/threads Core i3. But since you can OC Core i3, you can get even more performance out of it if need be,
comparison: http://cpu.userbenchmark.com/Compare/Intel-Core-i5-8400-vs-Intel-Core-i3-8350K/3939vs3935

Note: CPU cooler is 160mm tall and if your PC case can't fit it, let me know and i'll look for something shorter.

And as i said above, most games are using quad-core performance while some use single-core performance. Due to this, and if you want best CPU for gaming, Intel is better choice. Intel has been better choice for gaming for the last 7 or so years.
If you were to look back in the 2012 when AMD launched it's popular FX line, where FX-8350 (8 cores/threads) is still used at some parts of the world and if you were it to compare with i5-2500K (4 cores/threads) that came out in 2011, you'll see the same difference between those old timers and current Ryzen/ Coffee Lake CPUs. Where AMD leads by multi-core performance and Intel leads by single- and quad-core performance,
comparison: http://cpu.userbenchmark.com/Compare/AMD-FX-8350-vs-Intel-Core-i5-2500K/1489vs619