Again and again the Remington Express comes up as a good low-price air-rifle. I quite fancy one myself to enjoy exercising the diamond files and aluminium oxide paper on it.

However, I would recommed you purchase a secondhand fettled Weihrauch HW99S off one of the members of the BBS as you will then not have to upgrade it, and can start shooting with the best of beginners advantages, a decent trigger. They usually come with open sights and can be adapted to most air-rifle shooting purposes.

Here's one for example

https://www.airgunbbs.com/showthread...93-Hw-99-s-177