Funny, I was in a kind of similar situation not long ago. Online repositories are way too big to clone just for fun, don't bother.
For now, if your goal is computer literacy, I'd suggest you start with something simple but not quite braindead like Slackware, or maybe Debian. I'd nearly recommend Slackware wholeheartedly, but lack of information for essential dependencies in packages plus no internet connection really makes installing new software annoying. Anyway, just pick something that doesn't boot into a GUI immediately, figure out how to set that up yourself by following a manual or a wiki or something, and you'll have learned enough by that point that you can just figure out the rest later.
As for packages, I'd just install them as I come to need them. I used to have this tricky setup where I'd plug my computer into my laptop so as to use it as a kind of modem, but I don't know if that's worth keeping 2 computers powered on constantly. Is there no way you can connect your computer to your network directly? I got that sorted before too long, but I'd hate to have kept using such an inconvenient setup.

In regards to packages, you don't have to make a copy of the entire (!!) repository; if you want to install something, following one of the web search results for (something like) "apt-get download dependencies" should be sufficient (although I have concerns about keeping software up-to-date...); there's no way you could the first person in the world who needs to deal with a setup like this!
I also might suggest starting with a minimal Debian install instead (or perhaps not, depending on how computer-illiterate you are at this point; and experimenting with packages might be a pain without an Internet connection).

Thanks for the replies, and I apologize for the late comment!
I'll be starting work on the box this week after researching the dependencies for the programs I use. I was tempted to be lazy and use Ubuntu Studio, but that would defeat the point of me even buying another box to learn with. But since I'm already familiar with apt-get, I'll stick with Debian for now.

The box I have has an AMD chip, as I heard some good thing about those concerning Linux, but I may have been mistaken, going by how mixed accounts about them are. Linux usually does good on older hardware, so hopefully this won't prove to be an issue.

HP Anon again.
I'm still playing around with my system, and getting comfortable with the Debian OS. I'm still working offline, but was fortunate to run in to some useful commands/scripts to use the ISO files provided on the Debian site as repositories. This helped get the productivity programs I'm used to using easily, as well as testing stuff I don't use too often. I also went ahead and grabbed the Testing and non-Free ISOs to get some more recent stuff and AMD firmware. It seems I may have to jump in to compiling from source to get more exotic packages or games though. I've also read about apt-get offline, and that may be helpful if dependency hunting gets to be too absurd in some cases.

The first minor upgrade I had to make to the box is to get a video card. The integrated graphics unit couldn't handle Krita, and while it worked well enough for MyPaint, I didn't want to find myself running in to a wall productivity wise. At the same time I'm tight on cash, and could not afford to splurge on graphics units. So I went and grabbed an R7 240 low profile card, since it was under $100 and fit my computer. I later read/watched about how the card was terribad and couldn't run GTA V at 1080p. Fortunately for my case, I didn't invest in AAA gaming at all, so not being able to play Rise of the Tomb Raider isn't a problem.

The next few things I want to get for it are:
- a CPU upgrade, looking at the Athlong X4 845 for a reasonable amount of cores for graphic tasks.
- 500gb SSD
- Maybe some RAM. 8gb seems to be the golden number, but you never know.

I'm projecting the total cost of the computer and upgrades will come to $300. I don't think it's a bad hit, but someone more experienced could cut the cost more. I have to start somewhere~

>a CPU upgrade
What sort have you got now?

>500gb SSD
SSDs are a complete meme. Go for at least a 1TB HDD instead, it'll be cheaper. Reliability isn't a problem at all if you back up, and a shitty one should last for at least 5 years.

>Maybe some RAM.
8gb is generally considered the ideal nowadays, but as long as you get rid of the few hungry processes you'll probably have running in the background of this pre-configured system, you should easily be able to make do with just 2gb. Not even saying that to seem like some kind of hacker elite or anything, 2gb can be enough for most purposes, even emulated games going up to the N64/Playstation. The speed they work at could be a bottleneck though, if you can't run Krita properly. I don't know whether you can get small ones that work fast.

>>7
>What sort have you got now?
An A6-7400b.
I have a i5 laptop and it runs fine there, but ran slow on the PC prior to slapping a video card on there. Not sure whether the issue is the chip or the way the software is built, but I assumed I would run in to issues later anyway.

> SSDs are a complete meme
It's kind of sad that they seem near necessary to run Windows 10 (And another reason I cling to 7). I popped one in my Laptop and was charmed by how quick it booted up, but my Linux partition was already snappy to begin with.

I have a base urge to get all I can out of this old hardware. You can say I'm having fun tweaking things to my liking, as I'm doing such to the OS. I got a book/pdf full to info on the shell which I'm going to be reading a lot of too. I still a bit of a Windows kid so I can get lazy with the OS at times, but I legit feel excited about how much of the OS' vital systems are out in the open.

Greetings!
HP anon from a few months back. Since my last post, I've been slowly upgrading and playing around with the box, grabbing what I need based on the work I intend it to do with it. That is, until I fried the motherboard due to getting impatient while upgrading my GPU. The prebuilt itself was cheap enough that I could replace the box for ~$45, but instead I decided to stop being cowardly, and just build one using the parts I had built up over the months.

Specs:
ASRock FM2A88X Pro3+
EVGA 500BT PSU
Athlon X4 845
Radeon Pro wx4100
24 GB RAM (I had a lot of left over RAM sticks)
250GB SSD
3.5TB or HDD space total
DELL E2216H Monitor
DEEPCOOL Tesseract BF Case

I still have a few more components coming, but this box is close to complete. I want to use this box to start digging in to heavier multimedia creation, beyond drawing and in to animation and video editing. Nothing absurd like 4-8K resolution stuff, but I wanted to have enough computing muscle to have wiggle room with what I could do while not starting a house fire.
I did not trust myself to not destroy a computer when just starting out with a pre-build, but now I feel silly, since tue buiding process turned out to be easy. I would have saved a few bucks too. Still, it was a fun experience, and I'm glad I built this box. Hopefully I can keep working with it for some years before it becomes obsolete.

>>9
Nice.