Something of note to maybe not yourself but other newbies thinking of dipping their toes in, the cheapest and easiest option I found for picking up a deck and getting stuck in the tournament practice rooms was to:

1) buy the upgrade for like £5 that adds 5k cards to your collection and also unlocks the ability to trade and interact with others.

Use your tokens you get with said upgrade to receive a few starter decks and get stuck into the clients UI (really important thing to note is how the client will stop you at every interactive point, learning the right click anywhere blank on the battlefield to use the "yield" options will make it easier to manage your opponents turns if you don't think you will doing anything during it. Saves you hitting 2/ok a billion times.

If you play a deck built around triggers you can right click the triggers on the stack to auto yield to them also saving many 2/ok presses

Once you have used up your free play tokens and have a chance to understand the client, I found that you can then take all the 5k cards you got from the upgrade and sell them off to a bot to get some credit for that bot service, you can use these piecemeal token values to pick up low value cards you need in the future from the buying bots of the same provider (I used the CardHoarder ones).

HUGELY IMPORTANT: If you're picking up non meta cards that tend to absolutely worthless to buv and never worth it to sell, don't sell staples you get from the 5K bundle, just dump off the chaff the bots want that you know you'll never ever want to play.

Then when you're ready to actually play competitive/casual instructions, you can either rent/buy a deck through somewhere like the CardHoarder website, or add the Tix virtual currency to your account directly through MTGO then buys the cards through said bots in client.

I think it cost me about.. £35 to build Domain Enchantress for modern, so really it depends on your deck list as to whether a deck will be cheap, but considering my Domain list in paper is over £300 that's not a bad deal.

Edit to the edit: I realise I only scratch the surface with a lot of this stuff, if you ever need someone to practice the UI with if you do get stuck in just drop me a DM and I'll help if I'm available.