
“The best time to plant a tree was yesterday. The second best time is today.”
Officially - be the person. Be passionate and tireless. This isn’t about money - whips are cheap and if you can get some momentum behind socials then people will donate trees, tools and money.
Find what land your local council actually owns and visit it. See what’s possible. (No overhead lines, safe, easy access for pedestrians, soil conditions etc)
Then find out who’s responsible on your council and visit them. Make it easy for them - have lots of options, have the answers ready, make it clear there’s public demand. Most town officials have limited budget that’s already allocated, not much time, and are often not hugely motivated by yet another half-arsed proposal to spend both, so get it right. Be dogged, you’ll get brush offs and you’ll hear no a lot, so keep trying. Eventually you will connect with the right person at the right time and things will happen. The positive feedback then is pretty motivating and you’ll learn a lot of new skills.
Don’t expect universal support even from the public - a percentage of people will try to drag you down, or deliberately sabotage them. Forget them, or if they damage the trees, get the media in - the outcry will get you more momentum.
Obviously this is a lot of work, but it’s how stuff gets done.
If you can’t face that, I would understand it, and as guerilla gardening has already been mentioned, that’s absolutely a valid way too. Even just picking up rubbish on those walks is rewarding and beneficial.

I’m pretty happy with the asus vivobook I bought for half that. Very fast processor, 8gb, 256. My only criticism is it’s a little flexible in that if you lift by one corner the track button will fire. Minor issue, otherwise it’s been great for two years.
As everyone is saying “used thinkpad” - do your homework for the specific model. They are legendary, but that legend is not universal. Lenovo traded on their name and produced some pretty poor models for a long time. They are very often: Overpriced. Or crap quality. Or so old that they’re not usable.