As a low spender, the first thing is to be patient, which can prove to be extremely hard.
Before spending and regretting, you have to first be much more familiar with the game, and also see if you have more affinity with one specific sphere. I thought refess was the more suitable sphere for me at first, but the file/sphere I play the most in the end was Gowen. It took me a few weeks to realize it however.
I would not recommend buying the EX box, because you won't have other cards to use most of them. In fact, even if you decide on one sphere, you might want to think twice before buying EX9 for that sphere. The common pack can be bought without thinking, since even if you don't like any of the cards, you can recycle them all and get away without any Gran loss.
Prebuilts offer good basis for competitive file. By competitive, I don't mean you will take first place easily, but you'll be able to aim for >50% of wins, and 1600+ rp at least. Some people manage to do much better than that with each prebuilt (yes even for Gowen, there's at least one player that is 1700+ rp with it as a main file). It is one of the cheapest and easiest way to get a good file. And for your information, Sea Serpent is the most popular prebuilt, and is maybe the one with the most potential (some people reach top rp spots with it). Provided you like playing the file, prebuilts are worth their Gran (although I think they should be more like 1 k Gran for new players).
But truthfully, even a gowen starter with beginner packs and a few cards here and there can give you a more than decent file. Experience is an important factor to be successful in all cases.
As for sets, it's hard to tell you to buy a specific one. There are not many files you can build with most cards from one set (there are some, like shrine knights (SK)), and in that regards, set 5 and 6 are probably the most interesting. But I think it's best that you become more picky and determine what play style fits you, and only then come back and ask again.
Lastly, know that there are promotional offers on a regular basis. There is no need to rush and spend all your Gran now, so you can save some and be happy if by any chance some good offer appears.
I decided to stop Alteil because it is no longer fun thanks to set 10 and what some people became; I gave my account so if you meet me it's not me.