A few things to consider.
Nearly all smartphones, except iPhones, run Android. Android versions have a number, and a name that's a sweet treat. The latest is Android 9 "Pie" and before that was 8.1 "Oreo". If a phone comes with an old version of Android that probably means it's an old phone, which means it will have a shorter useful life. Phones tend to stop getting necessary security updates a few years after they're launched, and apps and websites expect faster phones as time goes on too.
If buying outright, IMHO the "sweet spot" is £150-250. Much cheaper than that and the phones become junky, with slow performance and bad screens. More expensive and you get premium performance but the value for money arguably falls away. (Of course it depends how wealthy you are). If you buy the phone outright you can get a SIM-only contract or just use pay-as-you-go, I prefer the latter because it means if I can't afford to top up I don't have to.
If getting a phone on contract, the monthly cost is basically split in two. There's what you're paying for your calls and data, and then there's the cost of the phone itself being spread over the contract term.
Google make the Android system, but then most phone makers tinker with it, some more than others. Samsung make significant changes, but they're so popular they're practically a standard anyway. This only really matters if you switch phones, or someone else is trying to help you (and can't find a setting because your phone has it in a different place).
As with most things I advise reading reviews in a reputable publication or website. Reviews written by people who know what they're talking about. You can't buy a phone solely on specifications. Specs will tell you, for example, that a phone has a 1080p screen but a decent review will tell you whether the screen is actually any good.