My dd is 11 and she's had a phone from about 9, my old one goes to her Dad, then gets passed to her. But she's only reall started using this year since starting secondary.
I think I'm lucky with her as I've not had to be strict or be hard with rules re screen time, she's used the family PC since about 18 months old and is very aware of safety, of in app purchases etc and won't even cross a pop up off without checking with me first. She only has people she knows in real life on whatsapp and tells me if anyone tries to contact her (one if her friends from school tried adding her and even though dd was sure it was her, she wanted to check with me first) she's allowed to download free games without asking, but she won't, she prefers me to check them first, as she knows porn can sneak in on pop up ads in the the most innocent of free games sometimes.
She leaves her phone downstairs overnight by choice, but sometimes takes it up if her dad is working away, he usually sleeps downstairs so figures if someone broke in to house he'd beat them up, but when it's just me and her she feels safer with phone in her room in case of fire etc. if she was playing on it at midnight it would be removed.
There was a thread about checking teens phones recently and it looked like a lot of people said it was snooping, but while I haven't needed to check on her, I have every password set and notifications etc for her sent to my email. She knows this and wanted it that way.
Also, while she knows not to talk to strangers, I've told her that if she ever feels like she's out of her depth with someone, or if she does chat to someone she shouldn't and they cross a line or ask her to do things then she should still come to me, she shouldn't feel she can't say anything to her dad or I for fear of punishment as I just want her to be safe.
You can monitor what they type into google search and youtube vids they've watched etc by logging into her gmail account on another device and going to google settings.
Dd is on three and we out 10 pay as you go credit on every 30 days and then use that to buy a bundle which gives her internet, texts and calls. There are some good pay monthly sims that you can pay month by month without a contract too.
You can make rules now but let your dd it's trial and that rules can be changed depending on how it goes, so if you met her have it in room shell know it can be stopped, or if you let her start with small amounts of screen time, once you know she can be trusted in can be increased.
Kids change as they get older so while I fell dd is good and self moderates brilliantly now, I know it coukd change and she could become sneaky or look at content I don't want her to, not just porn, but pro annorexic sites etc, it's a privilege she could lose and she knows it.
Also, rather than buying new, you could get a better pre owned model for same price at places like Grainger games. You get guarantees on what they sell etc. dd is wanting a newer one for Christmas (she currently has my ancient iPhone 3) and I think I'll get pre owned.