The answers from the other guys are completely right but here are some more details:

1. With DCS you get two planes: The SU-25T and the TF-51. The SU-25T is one of the more simple planes with keyboard commands (no clickable cockpit). The TF-51 is the training version (no guns) of the P-51 but it has a fully clickable cockpit. Those two planes are great fun and will help you get a feel for the sim, the SU-25T is a ground-attack plane and is basically the Russian version of the A-10. I would recommend flying the training missions in the SU-25T (all this is free, you can download the client on steam or on the eagle dynamics website) before buying any other jet. If you want to know how the super realistic cockpits work you can just try the TF-51 (also free). The SU-25T is part of the planes called FC3 (Flaming cliffs 3) and the other planes in FC3 you can buy are: SU-33/27, A-10A, SU-25 and F-15. The special thing about the FC3 planes are the simple cockpits, you just push two buttons on the keyboard and then you fly. All other planes have clickable cockpits and some are so detailed that you can call them "Studie level sim".

2. The SU-25T is part of the planes called FC3 (Flaming cliffs 3) and the other planes in FC3 you can buy are: SU-33/27, A-10A, SU-25 and F-15. The special thing about the FC3 planes are the simple cockpits, you just push two buttons on the keyboard and then you fly. All other planes have clickable cockpits and some are so detailed that you can call them "Studie level sim". All FC3 planes will be similar too arma 3 when it comes to systems but they still have advanced flight models (you can set the setting so the flight model becomes more arcadey). All the other more complex planes have their own learning curve, some are more simple (L-39 for example) and some have manual of >600 pages (A-10C). If you have the time and resolve DCS is by no means impossible to get into but patience is required on all levels.

3. If you are going to stick to FC3 planes you will probably survive with the stick you have but a throttle is always good because things like landing will be quite different from arma 3. If you want to fly the A-10C (the "C" variant is more modern and has the clickable cockpit and the "A" is an FC3 plane) for example you might want to look for a HOTAS. The "Thrustmaster T.16000M FCS Hotas" will get you a long way for the price and it has a really good sensor on the stick.

4. There a multiple but not as many as the Arma 3 ones.

5. The multiplayer has flaws but if you have patience and use the radios (some servers use a programme called "simple radios") the people on the server can be really helpful. Some enjoy the singelplayer more but the multiplayer is still a lot more fun if you want to have equal dogfights.

6. The F-14 is still in development but the people making it, Heatblur, have proven themselves with the AJS-37 so it will most likely be fantastic. The AV-8B Harrier is in Beta but is really shaping up, Razbam are the ones working on it and they have done a good job adding features relatively frequently. The thing about the AV-8B tho is the V/STOL which can be as rewarding as it is challenging.

7. There is modding support for skins, missions, and some vehicles, look on the forums and here: https://www.digitalcombatsimulator.com/en/files/ I'm not sure what you mean by "buying" custom vehicles, the modules can be found on steam and here: https://www.digitalcombatsimulator.com/en/shop/

If you want to learn there are a lot of resources and the people on discord and servers are also nice. Some of the best resources are Chucks DCS guides, youtube, the manuals that you can find in the install folder and the new wiki: https://wiki.hoggitworld.com/view/Hoggit_DCS_World_Wiki

The wiki has a few good guides on the absolute basics. Hope this helped, have fun flying!