SCAG seems pretty balanced, but don't allow flying tieflings (flying is silly to have early on).
I also advise not to allow anything from the UA articles. Sometimes it's not obvious, but there's heaps of broken stuff there. Just don't use any of it.
Be a tiny bit careful with multiclassing all round. At the very least, make your characters work for it a bit. How did the sorc end up with a fiendish 'lock patron? Why did the druid end up following a life god on top of his already lifey druidism, or take up the life of a monk? Even if the character concepts gel just fine, it's best to have character "progression" rather than just "levelling/building", even to the point of only giving some class abilities to some combos if you think the balance will be out of whack. Or let them "come online", half one play-session, half the next. Make sure your players know this at the start though, so they know whether the "level of badass" effect is or isn't going to work with that build.
And yeah. Be a bit careful about armour stacking. HP invulnerability isn't a good thing for a character to have. If you want to give out plenty of armour, just add +1-2 to-hit/AC for mook monsters every once in a while, maybe up to half the group of your player's party ends up quite powerful. Think of them as the elite part of the mook mob, but you have no intention of putting any real work into them. You don't want to overwhelm squishier characters, while still giving heavily armoured characters a challenge. +1 (or +2) mook-competency-bonus is almost like this edition's -/+2DC from 3rd. Actually, use -/+2DC as well, for reasons of situation or preparedness. Just assume that the bigger mooks try to engage the fightier characters, and that the rogue isn't always on his A-game unless he actually tries to be (saying "I do *this* complicated thing" isn't exactly prepared while combat is raging on, so they might get a negative to whatever they're doing).
Giving monsters and mook squads/leaders the equivalent of the Magical Initiate feat is also a great way of balancing stuff. Why does a wolf have Bless and Guidance? Because he does. He's the leader wolf. Until you knock the stuffings out of him, the pack is stronger and meaner. You can do this for any monster, for any low level spell, just tie it to the stat you want to if it's a spell that DC/to-hit matters for. It gives you a massive range of options for "balance" and provides better encounters overall. Chuck a lvl 1-2 spell or a cantrip or two on absolutely anything if you think your party will breeze through an encounter way too easily. Then there's plenty more to think about than just DPR and bonus actions. Try and make it in character for the monster, but sometimes it's just "because that's what they do". Spells for leaders, mostly just cantrips for entire mook squads. You'll get to know what your player's characters can and can't do pretty quickly, but adding MI to encounters from even pre-canned adventures makes the game "better" for you and your players, even if it's a low-magic campaign. It's low-magic for them, not for you. You don't have to cast the spells if it turns out you went a bit overboard for a particular encounter, it just gives you options. A lasso can be Thorn Whip, tricksy throwers can have Viscous Mockery, micro-mages can have Create Bonfire/stackable Ray of Frost. There's plenty to do that makes good sense and makes for far better encounters than most WotC written stuff. Think of level 1 spells/cantrips as a good reliable source of "not broken" options, and add them to encounters to spice them up a bit at will. Even if it's not particularly powerful, it does make it a bit more interesting for everyone if combat is getting a bit stale.
Combos to watch for:
Goodberry/life cleric (yes, HP can throw the game out of whack)
Moon Druid/monk (high AC and wildshape HP=bad for balance)
Sorc/lock (just a very good build, not broken, but very good)
Dipping cleric and not RPing squat to your god (cleric is an amazing one-level dip for most classes, often with no effort for the goodies. But cleric should come with RP baggage, at the very least)
Early written adventures tend to have weaker versions of the monsters compared to their MM equivalents. Feel free to buff them up a bit with the above options if you feel they're a bit lackluster. The starter set has less character options than PHB/EE/SCAG, so less potential for powerful characters, but I'm still not actually sure if the balance is better or worse because of it.