A good start is an Air Arms S400 with a big Nikko on top (both secondhand).
In time, if you get the bug, you can sell on the 400 and upgrade to an FT specific rifle.
The big Nikko is a great scope but is a little heavy. There are some other scopes to choose from but perhaps the Nikko is most readily available secondhand.
If you're completely new to FT you might be surprised to see that some folk will spend more money on their scope than their rifle. This does show how important the scope is for FT.
When you've set your budget don't forget you'll need a divers bottle and charging equipment.
Hope this helps