1. I have similar specs for my laptop. It’s definitely playable, even at fairly high graphics settings, but the frame rate is not that good and it tends to drop when your city gets bigger. Don’t know if it’s due to graphics limitations or if it’s because of the simulation.
2. I have mass transit, green cities and snowfall. For me, I think snowfall is the best expansion since it brings trams (and I love trams), but to be honest none of these expansions are really necessary other than for visual appearance. The vanilla transportation options are more than capable of dealing with all your mass transit needs. I haven’t tried park life or industries, they seem interesting but also looks like they require a bit of micro-management if you want them to be profitable so I’m not sure if I will get them. If I find a good discount, then probably.
3. Modding can have as big of an impact as you want it to. Mods that are continuously running, like Traffic Manager, usually have different options with different impacts on performance so you can just pick the option that suits your PC. Construction mods doesn’t seem to have any performance impact, typically they are only active when you are actually using them.
For user created buildings in the workshop you can see how big they are in terms of file size, I usually go for the ones that have a relatively small file size. There are also great collections of low resolution trees that look great (much better than the vanilla trees) and that can actually improve performance if you replace the vanilla trees on your map with them.
4. Economy is fairly straight forward. Might take an hour to understand. Traffic is rather complex and you have to use a “learning by doing” approach. However, I find that imitating real life solutions to traffic problems actually works well in the game, so if you build with inspiration from real cities then traffic will be pretty easy to handle too. The exception is industry transports which can be very intense, especially for cargo train terminals and cargo ports, so you need to be careful about how you solve their traffic needs. You can find a lot of good YouTube tutorials for traffic.
In my opinion the traffic management is the best aspect of the game, especially with the traffic manager mod. If you like problem-solving then this is the game for you!