editing SD footage doesn't really take much PC horsepower to do so you don't need to go all out, maybe a second hand mac/macbook/etc would be a good choice. I would stay away from anything that isnt in the i3/i5 region ie core 2 quad era as they're too old to be viable purchases. Second/third gen i5/i7 still holds up and is great value for the money. Since it's SD you can probably get away with 8gb RAM. Something with an SSD would be a bonus.
Here's a list of all macs that come with Firewire.
https://everymac.com/systems/by_capability/macs-with-usb-firewire.htmlWith that in mind, most of the Mac's around that price point will have intel onboard graphics which aren't terrible, but aren't great. Since it's SD it won't hopefully make too much of an impact but idk how well After Effects/Premiere would cope with a shitty gpu like that - Sony Vegas
should be fine.
Normally I'd not particularly recommend a Mac but since they have Firewire built in it's easier. When I had to use Firewire I had no issue with a random Firewire card off eBay, whereas my friend had to go through 3 before he found one that worked and even then it's a bit flaky. I will say that generally, if you looked for a used gaming PC around $300 you'd get better performance than an equivalent price point used Mac, then you'd just have to put a firewire card in there.
For example, barely looking and this is $350.
https://www.ebay.com/itm/Gaming-PC-GTX-970-Core-i5-16-GB-RAM-SSD-HDD/163784665955?hash=item262253ab63%3Ag%3A%7EmQAAOSwbXddMw8d&LH_BIN=1&LH_ItemCondition=3000For a used PC, 2nd/3rd gen i5/i7's are generally the way to go in terms of price to performance. Local selling pages will offer better results as opposed to ebay where the "gaming pc's" are usually refurbed office units with low power but capable gpus. Even then that would be perfectly adequate for what you want but I don't know how well those rigs would take a firewire card as they're usually limited in terms of expansion. To sum it all up:
- 2nd/3rd gen i5/i7 is the sweet spot for used price to performance.
- 8gb ram will do you fine
- something with a dedicated gpu would be nice.
- ssd's are always good.