OnePlus actually does a pretty fine job of supporting even their "older" models. The OP3 and OP3T were running up-to-date "stock" OS's not too long ago.
I remember the first one coming out (and haven't kept track of how many there have been) but I honestly don't think I do anything that needs more than what the first one would give me.
(I actually have a Motorola One, which is on the 'Android One' mostly-stock long-term upgrade committment, and was about £150 new 18mo ago? £100 phone or less would be plenty powerful enpugh, I just paid a bit more to know I'd get the upgrades, and thought the fingerprint reader might be useful. (It is.))