Areas where most beginners overspend and underspend?
Subjective. My opinion. Open to discussion. Take with a grain of salt. Overall, intended to be helpful for beginners.
Areas where beginners overspend
* Computer parts. It's 2019. Hardware evolved a lot faster than audio software. Most people aren't really going to need 32 GB of RAM, NVMe and Optane storage are overkill, 1000 watt power supplies for an audio rig are a meme and integrated graphics are fine. As long as it's not a really old desk or a laptop with very slow processor, it should be fine for most people's needs.
* Audio Interfaces. Decades ago, it was true that a lot of money was need to be spent in order to get good sound quality, but now technology has caught up and even entry-level options are fine for the home studio. $200-300 interfaces may offer some QOL benefits but in terms of actual sound quality a basic $50-100 interface is going to do the same job.
* Plugins. There are plenty of great free and inexpensive plugins. Some of the more expensive plugins may have some features for QOL, convenience, workflow, etc, but in terms of how they affect the sound, Parametric EQ is an EQ, and a Compressor is a Compressor. Stock plugins that come with a DAW are perfectly fine. At first I focused on acquiring lots of plugins, but once I realized how they worked, I stopped caring about acquiring more plugins and instead found more excitement in the actual progress I've been making using them.
* Microphones. There's a lot of marketing in this part of the market, and people follow hype instead of do blind tests, but really, there are great microphones at all price points. Perhaps the biggest factor which affects people's decision is the frequency response (or the overall tone) of the mic. Some prefer brighter, some prefer flatter, some prefer certain frequencies. A mic doesn't have to be at a certain price point to have the frequencies you like. Beyond that, recording environment, recording technique, the actual source material being recorded and the vocal fx processing will make a way bigger difference than what mic you use.
I say all this because many beginners will buy a Shure SM7B, Cloudlifter, Audient ID14, some expensive plugins and their songs still sound unprofessional.
Areas where beginners underspend
* Acoustic Treatment. This will greatly improve the sound accuracy of speakers and it will improve the sound quality of recordings by reducing wall reflections. It doesn't have to be Auralex. Cheaper foams can make a difference too if there's enough of it. DIY panels with insulation material are an option. Even blankets are better than nothing. It's not sexy to spend hundreds of dollars on foam or insulation material, rather than a flashy software or a fancy piece of gear.
* One-on-one time with sound engineers, mix engineers and other professionals in the industry. In many cases, you'll be able to get a lot of this for free. Make friends with them, find online communities with them in it free knowledge from them, ask questions. Ask them about your speaker and microphone placement (show pictures). I'm all in favor of doing this free wherever possible, but if there are professional that charge a reasonable price for a one-on-one session where they review your mixes and show you their workflow step-by-step, this can be helpful. To pay money and not get anything physical out of it may seem unideal, but in production / songwriting / anything audio related, the knowledge, skill and technique is far more important.