What and how much ( as little as possible) do I need to spend?

Well, TBH you can't go past spending $0 to get a feel for things first. It is 100% possible to get free software, free plugins and samples and learn how to construct beats and melodies using just your mouse and keyboard without spending a penny. I can almost assure you, if you buy a controller and software and spend money on samples and bloat you will create almost nothing interesting and eventually just quit and lose interest.

How much in seconds / minutes can I use from other people's tunes without being in trouble.

In all honesty, if that is a question you are asking, you aren't at the level to be concerned.

Hell, start off with a whole beat or remix a whole song and just learn how to cut and arrange things. I use to do that all the time just for fun and play it for friends. Just assume it can't be sold or used for promotion.

Spend a few years (yes years) first learning things then research copyright/free use terms. Don't expect to buy some gear and have some hits up on spotify in a few months that people will be diggin, tho the state of music these days who knows what people will actually like. If you assume the only one interested in what you create is you, and that doesn't phase you, you are in the right mindset.

I started off with completely free and open sourced software of the time, Fasttracker in 1996 then moved into "Buzz Tracker" by 2003 which is also completely free to use and supports VST plugins. However this is not "easy" software to use, it requires some hunger to want to figure things out and experiment, a skill lacking in a lot of today's youth.

It's Windows only and not at all what you would expect but is capable in the right hands and the price of admission is a good start.

https://jeskola.net/buzz/

These days you can try out something like Reaper which is also free to use with a startup nag screen and very acceptable licensing cost, more suited to the expectations of today's learners with a more familiar layout but again, not a simple thing to figure out.

https://www.reaper.fm/

As you progress and gain some knowledge what you need/feel you want will be much clearer, especially around if you feel you have the interest to actually continue. Making a "Beat" or an idea is super simple. Most of us could sit and come up with 50 or more interesting ideas a day. Where the skill and talent comes in is actually making something cohesive and finishing an idea, adding instrumentation and melody, progression and something that sounds cohesive, that is where many hit a roadblock and lose interest. This is all way before even learning about how to utilise effects, balancing instruments in a track, mising all your frequencies correctly, mastering, releasing, licensing etc…. There is an overwhelming amount to know if you plan to do it all and that is where a decent course would come in handy…. this all assumes you have some pre-knowledge of music to begin with, otherwise you may also want to learn how to read/write musical notation. Then there is mixing and production skills, completely separate barrel from writing and composing….

By all means, buy a controller, invest in software and plugins and load up on sample packs as most companies entice you to do, throw your money into it and have some fun if that is what you are after, nothing wrong there if that is what you want, but if you really want to get into it and have that drive and passion, you will be able to find ways to start off with nothing and build it up as you personally want. Look at many of the successful musicians out there, many started with almost nothing, maybe just a guitar and if they were lucky a 4 track.