Hi, Nic.
There are usually quite a few reviews and comparisons online - but it is a bit of a minefield sorting out unbiased from biased and sensible from stupid. So…
I’d divide the programs into three tiers - free; middling cost; and top of the range - plus you can get extras for additional functionality.
I think the best free one is GIMP, it’s downside being that it is less user friendly than many. I think at least a couple of restorers on here use it. If you can live with its learning curve there would be no need to look elsewhere because it is quite powerful. (Don't be put off restoring if you don't like it; many others are easier to use.)
The bulk of programs cost roughly the price of a tank full of fuel - depending on fuel costs where you live and the size of your tank! I use Photoshop Elements (PSE) which is a cutdown version of Photoshop. I’ve tried others but, for me, PSE has the right combination of features and ease of use. I think at least a couple of other restorers on here use it.
Then there is the full blown Photoshop which is still the industry standard, but these days you use it on subscription, making it a little expensive if your hobby is only repairing photos. I think at least a couple of restorers on here use it - one current and one who now only pops in occasionally.
Then there are various plug-ins and stand-alones for when your main program just can’t quite do what you'd like to do.
I did my first restore four years now. I use PSE for 99% of what I do. It could be 100% but I like experimenting a little with different tools.
The last one I did
https://www.rootschat.com/forum/index.php?topic=844219.0 probably took about six hours.
You need healthy eyes, patience, attention to fine detail, and a little artistic sensibility can help - I haven’t got much of that last one. When you’re learning, there are plenty of how-to online tutorials and books (e.g. “Dummies” series) for the better known programs.
Early in the piece, learn “Layers”. Learn what all the tools in your program can do. Use the archives on here to practice on, at the same time trying to see how others have achieved their results.
Good luck. And, beware, it becomes very addictive.
Peter