It's kind of ironic that Szell would be disliked by a modern conductor, because Szell was one of the first conductors who valued precision and correctness over expression and individual license. That makes him one of the moderns more than fitting into the league of Stokowski and Toscanini. Szell is a "safe" conductor because he doesn't extend beyond the letter of the score, which is good for a beginner. Later on, as they become more familiar with the works, more individualistic and expressionistic approaches might appeal to them more.
I think it's VERY important for someone's first exposure to a work to be a solid, well thought out performance. I had a few records in my early days that were mediocre performances... bad Brahms, bland Sibelius, etc... and they turned me off to those composers. I later came to learn that the problem wasn't the composers, it was the performances.
Itt's much better to start with a few really good recordings, than to throw your net wide and expose yourself to a bunch of crappy ones. Thankfully, the mega boxes, like the Living Stereo and Decca Sound boxes allow you to have your cake and eat it too. 50 fabulous performances for a couple of bucks a disk. Can't beat that.
Those are some good looking sets right there. Why couldn't you have opened with that recommendation? Instead, the thread got fairly derailed into something that the OP probably wasn't going for. Anyway Slayer I hope you have found at least something to look into. The living stereo set mentioned above looks brilliant.