Looks like the cage is bent or as Vantage says, been re-fitted cross threaded. A replacement would be about 20quid but you might be able to rescue it if you're on a budget, cages can be straightened in a vice or very coarsely, with a hammer. It's difficult to tell about the rest as it's so dirty, as has been said, use a degreaser to clean it all. Even so, a new chain and rear sprocket would be about £30 but you'd need some basic tools.
All of it is doable, it depends on your own skillset or your willingness to have a go. I wouldn't say you needed a new groupset, just the usual wear items and maybe that rear derailleur, possibly a front chainring, maybe some new cables but parts shouldn't be more than than 50-70 quid for all of that but you need to do the work .
All of it is doable, it depends on your own skillset or your willingness to have a go. I wouldn't say you needed a new groupset, just the usual wear items and maybe that rear derailleur, possibly a front chainring, maybe some new cables but parts shouldn't be more than than 50-70 quid for all of that but you need to do the work .