According to MacTracker the maximum OS version for your machine is MacOS 10.13.6 (a.k.a. High Sierra). There is no way to upgrade your present Mac so that it would be compatible with either Mojave or Catalina.
The most Mac for the money is probably the MacBook Air (starting around 900£) but then you would either have to live with a 13" screen or invest in an external display and those can get pretty pricey as well). However you could start with the MacBook Air and add the external monitor later.
With careful shopping (the monitor, keyboard, and mouse or trackpad being extra) you could get a decent Mac mini at roughly the same price point. The mini is a surprisingly capable system, can be upgraded/replaced piecemeal, and has enjoyed an exceptionally long support life span from Apple.
For the uses you described another Apple alternative at about the same price point would be the 12.9" iPad Pro and an Apple Magic Keyboard. Functionally it would do pretty much the same things as the MacBook Air (and maybe even faster), operate over either WiFi or cell network, use exactly the same Apple apps, and arguably simpler and easier to maintain, but you would lose the ability to connect a larger external monitor. (My son is doing some web design and development and does most of his work on his iPad because he finds it easier to manipulate code and graphics and the search engine rankings look at the mobile site because there are millions more mobile users.)
I forgot to mention that on this side of the Atlantic you can find some savings by buying factory rebuilt Macs. They come with the same Apple Warranty and have the same Apple Care eligibility as a brand new Macs. They are available through Apple's web site as well as through a few select re-sellers, but not the brick and mortar Apple Stores.
Last edited by joemikeb; 08/29/19 08:13 PM. Reason: Add Rebuilt Mac Reference
