I'm not claiming to know everything, as there's a fair amount of contention about the topic (One view presented by @Expositorjoe), but the idea of medieval life being a perpetual state of terror, danger, and starvation is mostly thought of as outdated. You won't hear most modern historians refer to 'The Dark Ages' for this exact reason; It was an idea proposed by people from the 14th century / Renaissance to prove their new cultural superiority over their predecessors, and the superiority of the Roman Empire that many wanted to emulate.
Most peasants, if they made it past about 20, could expect to live fairly long lives; Not as long as modern people, but generally longer than 40 years. The reason life expectancy was in the 30s-40s is because the way life expectancy is calculated includes infant mortality which was exponentially higher than it is now. Most people (Aside from in times of plague) would probably make it past 60. [1]
As for Clerical healing, that depends entirely on your setting. In the Forgotten Realms where every town has a temple, the answer is likely yes, life expectancy would either equal or possibly surpass what we have in the real world. There are some diseases we can't cure, but magic doesn't seem to have that problem.
Once again, Wizards depend on setting.
Same for writing.
In my settings, I try and run a sort of low-to-middling magic setting based on what I would call a simplified modern view of feudalism. A lot of historians hate that term (Feudalism) as it doesn't really reflect the nuance of the political climate anywhere at any given time, but everyone knows what I mean when I say it, so I still use it. This isn't an academic paper after all haha.
The way I set it up is as follows, assuming a country a bit smaller than England:
The vast majority of people life on manors. A Manor is about 1500 acres, inhabited by a Knight. The land is enfeoffed to him by a Baron. 25-30 households of 3-7 people will live on the lands of any given manor with an average population of 150. For every 600 acres of land a knight holds (Some knights could hold several thousand acres) generally speaking, the knight would be required to field a single Yeoman. A commoner armed and usually somewhat trained in combat, should his Baron call on him to fight. Most average Manor Houses will also have between 1 and 4 Men-at-Arms living in the manor.
A Baron will have about 15 manors, 1/3rd of which he keeps for himself, putting a sheriff or other official of some sort in the manor to tend it for him. One of these manors is likely a castle, in which he and his family live. A Baron's lands are granted to him by a Count.
A Count will have about 13 Baronies subenfeudated to him. A Count likely holds one or more of the baronies in his county, and the manors granted to him by those titles. His lands are granted to him by a Duke.
A Duke may have about 7 Counties, and like the Count, one or more likely belong to him personally. A Duke is granted his lands directly by the King.
A King will have about 7 Duchies as well, and though I'm repeating myself at this point, will likely have a Duchy of his own.
If you want to roll for this, it would go as follows:
Knight: 1 Manor, with 1d8 Fieldable soldiers (Comprised of Men-at-Arms, Yeomen, and/or a squire). Baron: 3d10 Manors (Each held by a knight. Some major knights may hold multiple manors at your discretion) Count: 2d12 Baronies Duke: 1d12 Counties King: 1d12 Duchies
A Kingdom built like this will have a population of about 690,000 over an area roughly the size of England. This doesn't take towns into consideration. D&D assumes quite a few cities, so we'll be generous and say 10% of the population live in cities. The total population boosts up to, let's say 800,000.
Roughly 5% of the population could be expected to fight. This is a gross over simplification, but it's easy enough. The number gets thrown around fairly often, and it lines up well with the numbers for a manor's Yeomen and Men-at-Arms plus the Knight, which would be 4% for a manor of 150 people. The extra percent accounts for household guards belonging to Barons, Counts, Dukes, and the King.
Assuming all that, this kingdom could field an army of 40,000. This seems like a lot until you realize that you can't field all the troops in your kingdom. Most knights are only required to server for a short period of time (varies depending on the requirements set when given their fief). On top of that, troops will be left behind to guard castles and manors from bandits. Maybe expect half of your total army. Probably less.
As for magic, in my world it's mostly uncommon at the manorial level. Sometimes a Manor or village will have a "Witch" which is really just a Sorcerer. Someone with innate magic, shunned for being different, but provided for by the people for the services they can offer. A Baron will be lucky if he has a low level mage with him, but Counts will have a Court Cleric or Wizard. Dukes will have one of each, as will some of the more powerful Counts. The King is the only one with the ability to charter towns and cities, so they fall directly under his rule, which means almost every Magic user does as well, as they tend to congregate in metropolitan areas.
The vast majority of the church are priests with no magical abilities. True clerics are looked at with absolute reverence; To harness the power of the Gods is quite literally divine, and people will respect Clerics because of it. Same goes for Paladins, though Paladins represent the law of the gods, which Clerics represent their ideas, so Paladins are like biblical angels in my setting. They're awesome in the original sense of the word, being amazing, but with connotations of fear because of their power.
Arcane users are generally suspect. They harness the power of creation, and in the eyes of the Church and by extension the commoners, they represent the hubris of man. Only the gods should harness the ability to mould the world, so the Church and the Wizard schools often butt heads, though Wizards have the support of the nobility. The nobility in my setting, like the real world, are in a constant power struggle with the church and have no personal issue with man having the same power as the gods.
Most houses will be made of Wattle and Daub. Clay and stick houses, framed from timber, which at first sounds rather unfortunate but they would have been passably comfortable. Excavations have shown that even farmers lived in decent sized homes, with room for the whole family. I don't have peasants knowing cantrips A) because my peasants distrust arcane magic, and B) because most just aren't smart enough to grasp spells. If they are smart enough, often times they don't have the resources to learn as like you mention they're likely illiterate. Well, semi-literate. They can probably read or write their own names, and the legal terms presented on declarations at the Hundred Court or posted by a Reeve, but not much more.
Sorry I didn't cite more, but it's late and honestly I made a lot of these tables and ideas a long time ago for myself, and never bothered to source them.
Citation: [1]