I have looked at all of your recommendations and adjusted my part list as below:
PCPartPicker Part List
CPU: AMD Ryzen 5 3600 3.6 GHz 6-Core Processor
(£179.99 @ Amazon UK)
Motherboard: MSI B450 TOMAHAWK MAX ATX AM4 Motherboard
(£99.95 @ AWD-IT)
Memory: Corsair Vengeance LPX 16 GB (2 x 8 GB) DDR4-3200 CL16 Memory
(£59.95 @ Amazon UK)
Storage: Western Digital Blue 250 GB M.2-2280 Solid State Drive
(£38.99 @ Amazon UK)
Storage: Seagate Barracuda Compute 2 TB 3.5" 7200RPM Internal Hard Drive
(Purchased For £0.00)
Video Card: Gigabyte GeForce GTX 1660 SUPER 6 GB OC Video Card
(£214.44 @ Amazon UK)
Case: MSI MAG FORGE 100R ATX Mid Tower Case
(£49.99 @ Amazon UK)
Power Supply: Corsair TXM Gold 550 W 80+ Gold Certified Semi-modular ATX Power Supply
(£69.98 @ Amazon UK)
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-10-04 03:34 BST+0100
I'm considering my options of using the 2600X along with the ASRock B450 Pro. Ideally, I would like to pair the ASRock B450 Pro with the 3600 but I've been looking at the compatibility and the MoBo will require a bios update out of the box. I don't have an old CPU which I could do this with.
Also, people have mentioned AMD are going to announce their 4th gen soon so maybe I should wait and see how the 2600x will hold up against these and against the 3600. I don't plan on buying a 4th gen as I am building a budget PC for my brother, I'm just trying to squeeze the budget as much as possible to increase longevity i.e. so that in the future he could just ask for a new GPU instead of having to upgrade several parts of his PC.
I switched out the PSU based on recommendations but I will definitely check that one you've linked out... Maybe for my own future build aswell!
If you had any final comments it would be much appreciated.
The issue here is you mentioned the word 'budget'.
In that respect the 2600x is a better buy imo irregardless of it being older and slightly weaker.
Its around 12% less powerful on multi core at the max, it costs 33% less, it also has a better stock cooler supplied.
It also doesn't suffer any of the rapid voltage and temp spikes that the ryzen 3*** does which tends to facilitate the need for an aftermarket cooler if you don't like whiney fan noise.
Don't get me wrong here, the 3600 is a great cpu but the 2*** cpu's should not be ignored at their current price point, they're still absolutely relevant performance wise and an absolute bargain on a budget.
The point is it will suit your brother absolutely fine, it'll run a gpu upto something like rtx 2070 super or 5700xt levels absolutely fine.
If your running anything less than a 100htz screen you're not going to see the slightest difference between a 2600x and a 3600.
If you really want to spend £180 on a cpu with longetivity and future proofing in mind then buy the 2700x instead.
Irregardless of age and slightly lower single core performance it's way better than the 3600 overall!
You could also run it on the asrock board no issue at all pulling £20 of the budget back.
I'm running a gen 1 ryzen 1700 here with a 5700xt, it has no issue at all powering that card and the 2700x is some 15% stronger.