I'd recommend a Wright Stander 36". I know it looks like a lot of money, but they're fast and will earn back the extra cost and more. The model with the spacer height adjustment is about $4,000. You could pay your way thru college with a machine like that.
The 36" looks kind of small, but they're so quick they make up for their size. Throw away the 21" and get a Stihl FS 110R trimmer and a good backpack blower like a Stihl BR600.
It's now closer to $5,000 but spread that equipment cost over the next 5 or 6 years and it looks kind of small, especially compared to the money you'll be able to earn.
Plus it's small light and compact enough you could trailer it with a car and save the expense of owning a truck.
If you were my grandson that's exactly what I'd recommend, but my grandson doesn't show the ambition you do.
If you're looking for financing don't forget dealer financing. They might be able to work something out.
Good luck to you.