While this would be better posted in the current elona thread, i'll do my best to help you, though i've not played much on Elona recently or slept much for that matter, so this may be a little inaccurate, though nothing that won't point you in the way that you "need" pointing in anyway. i'm sure someone'll help by pointing out where i've made mistakes, anyway.
First things first, the missions are hit and miss as you've noticed, but some towns get more of certain types - that as far as i'm aware is more harvesting in Yowen and more Performance ones in Palmia. Beyond that, i don't think there's really much way of getting more escort type missions beyond just checking each towns board when you arrive.
The issue with dying over and over like that's something you'll run into, but if you train up your character with things that increase Con, that'll help - though as anything bar a warrior type, for the most part, i'd recommend dual shields, a laser pistol and decent armour (best kept 'Light' as a mage to avoid more casting penalties, that you can check what level of armour encumberance you're at in your character sheet.) that given you're a mage, will lower your spell chance a little if i recall correctly due to the shields in both hands.
Shield Skill + Healing + More Con based skills being trained = More HP and more Defence, after all.
Spellbooks, as a mage can be obtained by joining the mages guild in... Lumiest is it? and then giving them the spellbook that you wish to buy more copies of, or something to that effect. Other than that, you can stair-scum in the puppy cave to get spellbooks by going up/down floors repeatedly and grabbing books near the stairs, or, you can invest in a shop and increase the level of it, that when done with the magic type vendors, means they can stock higher power spellbooks. Mages guild still, assures you get a specific type, from what i hear.
Harvesting/Performing i'd not worry about until you're able to hold your own and perform quests/clear easier dungeons without too much issue, as by that point you can start gaining the platinum a lot more readily to buy and train up the skills.
Gardening and Strength i believe increase the speed with what you can uproot plants in harvesting quests, but it's always best to clear the immediate area so you can havest in peace, otherwise you get interrupted and have to start pulling the item up again.
With your armour, the first number is the evasion bonus the item gives and the second the protective value it gives, thus, your [5,1] hat gives a +5 evasion modifier and +1 protection modifier. Blessed items give +2 to protection and i -think- evasion atop that, unless they're weapons, in what case it relates to hit/damage.
With your weapon, the 4d3 is the hit dice, the +1 is damage added after the dice roll and the number in brackets after that is the hit chance. if they're square brackets, as with defender weapons and the like, then the weapon also adds protection.
with your stats, the number in brackets after the first number is your base skill, i believe, where as the first number can be higher/lower due to buffs/drains in effect.
The 0.8 after your melee figure is your damage multiplier, that's based on a combination of Tactics and Strength - you, as a mage do not have tactics, and so the 0.8 means you're doing 80% damage. The same goes for your distance skill, only it's a different skill (marksman) for that and i'm not entirely certain if it's perception or agility or anything at all stat wise that adds a touch more to that multiplier. the hit part is your base hit chance, against an enemy with zero evasion, that means later enemies can still be missed with 150% hit chance. protection is your armour's damage reduction, and evade is your base evade chance, vs an enemy with no bonus hit chance.
Life 80, refers to you having 80% of the hitpoints a 'normal' person would have, where as Mana at 140 means that you get 140% of the mana, in this case. the bracketed 145 means that you've got (most likely from a perk.) another 5% mana capacity atop that. I'm not sure of the exact mechanics behind how much magic/hit points these two translate into based on level and stats however.
your also overweight with your cargo, that means that you'll move much slower on the overworld map, so keeping that under your limit's a good idea - anything up to your limit with cargo items gives no penalty after all.
finally, no, you need to spend platinum not just to learn the skill, but to also increase your learning rate of it, as you buy it at 50% rate.
hope that's of help, explaining those things for you.