I'd like to upgrade a TP-Link TL-WR841N v7 to something that can run OpenWrt. The TP LINK works fine (except for not being able to run OpenWrt) so I don't need anything fancy, just something that would run OpenWrt without problems, now and in the future.
If you are not looking for something fancy, then you could just see what routers are available for good price, and then you check if they are supported https://openwrt.org/supported_devices and also check their specs in https://wikidevi.com
As for TP-Link, there is Archer C50 and Archer C7--you could find them used too.
And a good bargain usually if you are on budget is to look for ISP-provided routers (for example, if you are in the UK you could get BT Home Hub 5 Type A form ebay for 10 pounds!
No need to go to wikidevi for that, we have the same information in the OpenWrt wiki.