If you can purchase one of the Pre-built files, that can be a really good option for a number of reasons.
1) there are a few ways to get free Gran in the early game, like the Interactive tutorial, recycling cards from the 100% treasure battle rate in Crest (beginner's-only arena), etc. so you should be able to get one for very little real money.
2) most users have experience with the pre-builts, so it's easy to find advice or example battles to watch. This can really help learning the game -- both learning how and when to play your units, and also what tweaks are good for the files.
3) As you learn to play and tweak your pre-built, you should be able get a good number of wins because they're solid files. More wins means more free cards from treasure battles, and more weekly allowance gran. In other words, it'll be easier to get more free Gran in the future. Then if you want to build a different File, it'll be a lot easier.
"Scissors are overpowered. Rock is fine." -Paper