what is your budget? for hardware, I would either recommend a used laptop, or some entry level laptop. For a scale, I bought something for 500$ USD (did not buy in US, converting pricing to it) and for that I got - a recent-ish 8 core cpu, with a good (relatively) igpu, 24 GiB Ram, 1 TB storage, and 50Whr battery. If you want to target for 300$, you can roughly halve these specs, and get reasonable deals.
For distro - Anything works. If you want something to set and forget - pick a immutable distro like Silverblue. All distros essentially will act same, don’t try many. What you may want to try is Desktop environments (you can either check youtube videos, or project webpages). If no windows is installed, manufacturers either put opendos on it, or just plain blank, in either case you would prepare a installer media (instructions available on all distro websites, it is easy), go to bios, allow usb booting, then plug the usb, while booting select the media, and start. Then most installers are guided enough to help you.
Best of Luck.
Going for 6-7 years - you need to keep a few things in mind
depending on usage - battery would have to replaced once or twice - so machine has to be somewhat repairable.
a subset of above - 2 or 3 cleanings per year
also try to find repair guides for your device, the harder it is to repair, less good of a deal it is. For my old laptop, i have to not only remove all of the motherboard, but then de-rivet a back plate, and then replace keyboard and re-rivet. This forced me to buy a external keyboard.
go for a good display - at this price, tn is very common, my old laptop was tn, and it sucked. go for IPS. (difference in price would be $10)
if you are flexible with budget, don’t buy really old stuff. Anything from last 3-4 or even 5 years is fine, but any older is definitely not. since you want to future proof, going for maybe $100 more for a better cpu may even pay well, if you amortize the cost over 4-5 years
go for something not plastic body - my last laptop was plastic body, and in nearly 2 years of daily commute and medium to heavy usage, all edges broke, and in 1-2 falls, i even had display bleed. Unscrewing and screwing back made plastic bits fail over time. If you can find a metal body, then great, if not, try to get something which has parts of body metal reinforced (my current one is this)
whatever hardware you get - please check linux compatibility beforehand. for laptops this will include keyboard - touchpad, wifi cards, and so on
do not go for a gimmicky device
Don’t be overwhelmed by linux. These days, operating systems are anyway glorified boot loader for browsers, where people do everything. If you need any help, post in communities and you will find help. You can also message me if you would like that.