You'll be lucky to find anything that exactly fits your requirements.
The problem with the ROM scene, especially Lineage (and, formerly, Cyanogen) is that the developers of the ROMs tend to go for the name brand devices and the higher end ones at that.
The reasons are varied but principally it boils down to the fact that people who spend a lot of money on a flagship device are the ones who will either have enough money to simply replace it with another expensive one a couple of years later or else don't and, therefore, want to eke the most they can out of the one they buy first.
People who buy cheaper models either do so because they aren't tech savvy and/or because they aren't rich and, therefore, what they tend to do is get a 24 month contract with a 'free' upgrade at the end of it, if they extend their contract for another 24 months - so, they don't want/need a ROM.
So, it tends to be those who aren't rich but are tech savvy who develop ROMs, so that they can have a top end device and make it last a couple or three more years, so that they can spread the cost of it over time.
If you want something that will take a ROM, you're best off going to the developers/maintainers of the ROM you're interested in, seeing what models they support and getting the cheapest one you can.
But, it depends on why you want it.
If you just want to play with ROMs as an educational exercise, or you want to use a specific ROM, then take my above advice: find the developers/maintainers, see what they support and get the cheapest one you can.
If you want to prolong the lifespan of what you've got (or will get) rather than use a ROM specifically then you have three routes:
the above mentioned one
getting something with an unlockable bootloader and rooting it with Magisk (and possibly microG).
getting something more expensive but spreading the cost over a number of years, so it works out cheaper over time - the Fairphone (fairphone.com) is an example of that latter, with user replacable parts and continuing OS updates for at least five years (or that's their goal anyway) ... but you could also take a look at the Nokia 2.4 or 3.4
I've used Motorola Moto G range devices for years myself because they're good mid-range devices for not too much money (around £239 instead of the £800+ for flagship), have pretty much stock Android and an unlockable bootloader .. but I have to get the most expensive of them (the 'Plus' model) if I want any hope of finding even instructions for rooting them, let alone finding a ROM that will work with them - things have improved vastly with Magisk, but finding ROMs guaranteed to work with them still isn't a simple matter.
Bear in mind re the ROMs that, if what you're trying to do is safely extend the lifespan of your device then it may cost you more than you'd like because you need to go for the ones that the developers put the effort into maintaining security updates for - and those tend to be the more expensive models. So, you might be as well off simply rooting a cheaper device and maintaining it yourself insofar as you can - once the OEM stops releasing security updates, it's down to you to lock it down ... and once the apps are no longer updated by the app developers, no ROM is going to secure them for you anyway.