Apart from v1, all of them should be reasonable to use - obviously more flash (v4, v5) is better, but so would be 128 MB RAM and a second (5 GHz) radio; that said, you're unlikely to find anything but v5 in the current market anymore.
But I'd still like to echo Bartvz and suggest a more reasonable device for buying in 2021 (concurrent dual-band, more flash/ RAM - and 802.11ac instead of 802.11n); mt7621 and ipq40xx come to mind.
Thanks for clarifying Peter, that is the thing, I am a total newb, I have a specific application and with the 1043 it just works, performance is ok but with two superior routers I tried they just do not work.
So when getting one for a friend I know I can backup mine and copy to another and it will just work, I may even give them mine and start with the hopefully better one.
I think I have a V2, it cost me less than a couple of cups of coffee, I have seen several on eBay for various prices but not that cheap. I will probably hold out until a V5 comes along, currently most are V1 and a few V3.
I have bought other Routers only to find that they have issues with OpenWrt or they just do not work, so I am going to put them back on eBay and treat myself to a decent 2nd hand router.
The R7800 seems good but their prices have gone through the roof, mad prices being paid.