The issue with equipment availability is because the 4 basic classes were the only ones available until about a year ago - so there are 7 years worth of items intended for those classes, compared to 1 year and a mass update query for the new classes.
As to what is best for soloing, in reality, there are areas designed for every style, so it's more a matter of finding the area that best suits what your character can do. However, there are certain things that are pretty consistent.
Especially at higher levels, the xp calculator is heavily weighted so that a mob that has low hps but does high damage will be worth more than mobs that have a lot of hps but don't do as much damage. With some, you want to stab them as hard as possible to be most effective, but with others, you want to blast them as hard as possible. To do both of those, you need thief and mage classes. The monk class is designed to not stab (deathgrip) as hard, but the damage is reduced, allowing the fight to continue longer. If you are soloing and hitting mobs, you can pretty much assume that they'll hit you back, and you want to be able to heal yourself or gaining xp will be insanely slow. That means you need cleric in your classo.
My personal opinion, is that to solo effectively, you do need mage and cleric. If you aren't mage prime, you need either thief or monk (because mage primes get images, so can work around stabbing). If you're taking monk, I'd take it high in my classo to get the most of my open hand damage and whatever other skills are classo dependant. Other than those, I'd take whatever class entertains you the most, or has the skills or spells that you think would best help you.