Exactly as the title says. I'm determined to finally build a PC to at least get my own materials captured and do some editing. Ideally I'd like it to be able to double as a HTPC so I can watch projects without burning discs and for it to be able to run some games here and there.
Primarily I need to capture my LD collection. I've now got over 500 discs so that's a lot of audio detective work to do.

I've set aside $400 so far. Would that be adequate to build a decent editing machine for our usual line of work or is it going to take a lot more? I've never actually done any PC building before so I really have no idea what I'm doing.

I'm pretty happy with my brand new used PC https://fanrestore.com/thread-1312.html

It has a very good price/performance ratio. Probably you can grab something similar for a similar price...

Depends what you wanna do, but $400 is very little for a solid editing PC. May be good enough for SD content, but you probably won't have a lot of joy editing HD content I think.

I have computer that's about 4 or 5 years old and still holds up well, but it cost me about 2000 EUR back then. i7-3930k, 32GB RAM. Recently upgraded with a GTX 1070.

You will be able to find good video and bit-perfect audio capture cards used for reasonable prices, although you will find that some of the older devices will only have drivers for up to Windows7/8 so it's worth keeping in mind depending on what OS you go with.

Regardless of what system you choose, you will need storage! Lots and lots of sweet storage space. Enough is never enough, like gases projects expand to fill whatever space they occupy.

I'm building one for my girlfriend and I understand your situation, a few years ago I was in the same situation so take my words as advice.
If it was only for capturing audio from Laserdiscs then YES, you could do that even without an actual computer, but if you have to edit too (let alone play videogames) you have to spend easily 2000 dollars! Maybe "only" 1500 if you don't plan on gaming!
Editing (=rendering) requires 16GB of RAM (that alone costs 200 euro here in Europe these days) and an i7 processor (another 300 euro at least) so memory+CPU alone are already way beyond the budget. You need to save more money but also prioritize your needs (consider that since I got into editing I never had time to play anything, videogames are a distant memory so graphic card didn't need to be expensive ).

So, I was lucky to get mine for "only" 550€! About games: Freecell (more than 500 games, 100% win!) and Candy Crash Soda Saga - both included with Windows 10, and both addictive!!!

RAM memory doubled in price this year and decent graphic cards skyrocketed because of the bitcoin miners. It's a bad moment to build a PC.

When Bitcoin fails, I'll be laughing and saying the industry had it coming.

Logical Increments - How to Build the Best PC for Video Editing

"This might sound counter-intuitive, but the graphics card (AKA video card) is a less important component when it comes to video editing and other creative work with most software. Compared to the CPU, it’s usually OK to go a little cheaper with your graphics card. The exception to this rule is editing with DaVinci Resolve. If you plan to edit with Resolve, you absolutely need a powerful graphics card, as that software is powered primarily by GPU instead of CPU."

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4K Shooters - Building an Insanely Powerful 4K Video Editing PC for Just $998?

"But if you’re already wondering just like me how this could be even possible, you should definitely check out Max’s recent video below that showcases the out of this world capabilities of the amazing AMD RYZEN 4K video editing PC that not only stacks up against his previous Intel’s custom built machine but even manages to deliver a better performance for a fraction of the cost."

With my tax refund I have a bit more to work with but still even $700 seems pitiful in comparison to builds I've been seeing.