Hi Colin, lovely to read of a post Vatican II young man who embraced the grace given to you by God to be a better Catholic.
To begin with, a little wisdom of experience. When it comes to asking advice, or reaching conclusions as to who or what is valid these days, only God knows this. As I or others are not God, I do not make such decisions, rather I take it that they are valid until heaven lets us know otherwise. My advice then is to do as your SSPX priest advised you, advice you posted above, do not judge any traditional or NO Mass or priest or sacrament as invalid. I have always held that if the intention is there God will not reject it.
That said, there are two aspects of one's choice that must be considered. What one might hear at any particular Mass or church, from the pulpit that is, and what those around you might be saying. That is where the danger lies. So first of all Colin learn to know your Catholicism. Given you are so young you probably did not get the catechesis necessary to know all aspects of the Faith and could be led astray in so many ways. The second most important thing you need is to be among like minded traditional Catholics who do know their faith.
Now in my experience I have heard Modernism in both the NO and indeed at the Indult once or twice. Their Masses may be valid but their messages may not always be Catholic. There may be occassions where one might attend a NO, for a funeral for example, that is OK with me. So, as it seems to be in your case if you have a SSPX chapel near enough to you, that is the safest bet for all your needs these days. Make friends at these Masses, and together you will keep your faith traditional. I have heard of some cases where things are not 100% in some SSPX chapels, but thank God not in mine. Individualism can cause this, but I have not met one yet. But more than that, for as in your case some day you may meet a lovely Catholic girl you would like to marry. I have see such couples meet, marry, and are still attending the same SSPX chapel with their children, God keeping them very happy.