I want to corroborate @tbutz‘s mention of chromebooks. They are very cheap and surprisingly sufficient for what most people do on their computers in the classic notebook form factor.
With that said, I’d also second @Eden’s recommendation of an iPad. This device has come a long way over the last decade and has matured a lot both in hardware and software. We’re at a point now where I have no problem recommending even the lower end models to most people as their primary of only computer. iPad will naturally work harmoniously with your iPhone too, which is a significant point in its favour over a chromebook. They’re also quite expansive with accessories too.
iPads are to laptops what cars are to trucks.
If you rely on this device for your job, you’re going to want to spend a little more up front if you can. It pays off in the long run. The type of device will want in this case is dependant on the nature of the work.
MacBook Air is a fantastic all rounder and one of the best value products on the market for its price, but this is significantly beyond your budget.
Microsoft Surface devices are quite versatile too and have a lower entry level price than laptops. The only downside in my opinion that’s it’s a tablet first device running a desktop first OS.