You sound much wiser and mature than most 17 year olds. You have reason to trust your judgement, from what I've seen you type. The 67-72s, although not all that much more, are definitely simpler and a tad easier to work on, overall, especially a basic model like this C/20. I say go for it. It will have greater resale value, as is, than your truck when time comes to move on.
Show us pictures of both. Show a lot of interest in the C/20. Look it over hard. You haven't told us all that much about it. I am leaning toward saying go for it. It's always better to love what you own when the money doesn't come easy. You will have something you 'want' to spend money on. You will have more money as time passes. The low gears don't matter because at your age you are commuting out opf town everyday or traveling far and wide. The stuff I drove when in HS I never considered driving hundreds or thousands of miles out of town.
Like I said, go back and show a lot of interest... more than once. Let him know, casually, that it's not that you don't have the money that you are hesitant. It's just a big step for you being in school, where you live, etc. Talk a little about your job plans. Just show true interest and hopefully the seller isn't a dick and that will mean something to him. If you know you are going to do this you should make a reasonable offer, like $3k. That is a very typical offer and he should not be annoyed by it. Not just an offer, tell him you can give him $3,000 and stand by that. If you simply ask if he'll take that much, when he comes back with $3,300 where do you go from there. Show you are firm, have a clue about what you are doing, as well as support from many others with vast experience in these trucks. Many who are in your economic region. Most of all, always be willing to walk away. Never act like it's the last truck on the planet.
I have never owned a vehicle that I didn't think was cool. Never had a hard time finding my rides in the parking lot. And they were almost all older vehicles, and most I didn't spend that much money to buy. I made up for it with work.
Your parents are strict? Give you lots to do around the place? Show them you are willing to take on more, partially because of this truck, in the way of getting a job while keeping up with your chores. If you folks are as smart as they are strict they will see this is a great thing for their son in this world of lay around thumb controller teenagers.
I say go for it... carefully and wisely. You can do it (and you'll never beat my record
)