MattLong201 wroteI have a Samsung 32" LED monitor (1920 x 1080 resolution).
TV or monitor? If it's a TV, dump it and buy a nice monitor. If it's a monitor, use it for now at least, but I'd probably still want to upgrade it at some point in the near future.
MattLong201 wroteI haven't planned to overclock, not sure if I should?
Not if you don't want to. There's no need to overclock unless you specifically want to really. Well, with Intel. With Ryzen you really might as well OC IMO since all Ryzen CPUs are unlocked anyway, and it's really not hard to OC.
[quote=MattLong201"]Is the motherboard "meh" because of the brand? How do you know what to look for in a good mobo?[/quote]
Not the brand specifically, no. Companies make good and bad products. There are different things which will determine how good a motherboard is, mostly the VRMs. Gigabyte's VRMs are meh. They're not god awful, but there is definitely better. AFAIK, all of Asus' AM4 mobos are solid, so I tend to lean towards Asus. I'm a bit bias towards Asus though because I've used Gigabyte and MSI mobos in the past, but my current Asus board is so much better, and Asus' UEFI is the best you can get IMO. You're really not going wrong with any B350 or X370 mobo, aside from a few, like some of Asrocks cheapest B350 boards are utter garbage.
MattLong201 wroteAs for the PSU, is it iffy because of the brand or the capability? (i.e. if I up it to a, say, 700W is that better, or is it because I'm getting a Thermaltake?
No. Same as with motherboard. Companies make good and bad products. Thermaltake have some pretty decent PSUs, just this specific one is pretty bad. Again, it's not the worst but you can do better. As for wattage, a 450W PSU is enough for any modern single GPU system, as long as you don't have an HEDT CPU(like Threadripper or X299).
MattLong201 wroteChanged case to a Phanteks Eclipse P400S (https://www.pccasegear.com/products/37220/phanteks-eclipse-p400s-tempered-glass-satin-black/) .
Super solid option. This is what I was going to recommend. Although, you could get the P300. It's the P400S' little brother, so you lose some stuff, like rubber grommets for cable management, you lose some fan/rad support, and the P300 is a bit smaller but it's $50AUD cheaper, at $80AUD. It's a bulletproof option for the price, but the P400S is better so I'd be choosing between those 2.
PCPartPicker part list / Price breakdown by merchant
CPU: Intel - Core i5-8400 2.8GHz 6-Core Processor ($279.00 @ Scorptec)
Motherboard: ASRock - Z370 Pro4 ATX LGA1151 Motherboard ($195.00 @ PLE Computers)
Memory: Corsair - Vengeance LPX 8GB (1 x 8GB) DDR4-2666 Memory ($105.00 @ Umart)
Storage: Samsung - 850 EVO-Series 250GB 2.5" Solid State Drive ($132.50 @ Shopping Express)
Video Card: Gigabyte - Radeon RX 580 4GB Gaming 4G Video Card ($369.00 @ Shopping Express)
Case: Phanteks - ECLIPSE P400S TEMPERED GLASS ATX Mid Tower Case ($129.00 @ PCCaseGear)
Power Supply: Corsair - CXM (2015) 450W 80+ Bronze Certified Semi-Modular ATX Power Supply ($75.00 @ Shopping Express)
RX 580 is going to be more than adequate for 1080p. If you up the GPU to a GTX 1070, you'll be right around the $1500AUD mark, but I'd probably only do this if you plan on getting a better monitor sometime soon. Coffee Lake i5 is by far the better option over Ryzen considering you only plan on gaming. Unfortunately, Z370 motherboards are the only ones available right now that are compatible, so you have to pay more for a mobo than I'd like, but even with this added cost, Coffee Lake is still going to be the better choice than Ryzen.