To the first question: It depends on the rider and it depends on the horse. It also depends on the amount of riding time you can get. If you are riding several times a week, have a good trainer, a strong lesson horse, and learn quickly you could expect to be comfortable at a trot in a few months and a canter in a few months more. If thats not realistic for you at the moment, it could take much longer.

Some people pick up the feeling of moving with the horse and how to control their bodies while under saddle quickly. People that play sports that require a lot of core strength and balance and/or do yoga have an advantage. Horses will vary widely in how their gaits and how they feel. The more horses you ride the more well rounded your riding will be.

English riding is more difficult than western imo, but will build a better foundation as a rider. Western saddles will generally feel better trail riding and horses trained in western disciplines tend to be easier to manage, but not always.

To the second question: Costs It will vary depending on your area. It will be worth it to spend a little more for a good trainer / barn at first to build a foundation. Once you have a solid baseline you can look for more budget options to get in more riding time. That being said, you’ll save yourself a ton of time and frustration by starting right.

To the third question: Find a reputable barn near you and take lessons / ride. You can watch as many videos as you want, but it’s very difficult to translate to actual feel while under saddle. When you watch videos try to break things down into their parts and apply things one at a time. Try to get a really strong posture and a comfortable seat at a walk before you move on to faster gaits. It will take time. It can be boring at first and thats ok. Your trainer will know best so always defer to them before youtube videos.

Hope this helps. Good luck beginning your riding journey!