Welcome to the sport that is
WM
Having a range themed army in Menoth is definitely possible, I just hope that magic is part of said "range." Severius is a very good support/ranged caster for he prefers to sling spells through arc nodes for maximum effect. To expand from the battlebox and you decide on Severius, I would keep the original arc node and grab Severius' character jack. One unit to consider would be Errants. They are pretty good at range (especially with help, ie
UA/Covenent/etc) and are not useless in melee.
As for paladins, I almost never leave home without one and vilmon in a list. Vilmon is very point efficient for at 3 points he can take down a jack/beast himself and is hard to deal with when you throw impervious wall into the mix. I find it often best to just have him advance in impervious wall into melee range of your enemy. Most of the time your opponent can't hurt him and if they try to leave that's a free strike they will feel (P+S 15 +
WM + Boosted). If you plan to sit back/play static and just sling spells/shoot, paladins are a good choice due to stone and mortar stance will keep them around most cases.
You are correct that some Menoth casters are very squishy. Because of that you may wish to consider a Devout as your caster defender.
So order of purchases to start
IMO would be:
Warcaster (Severius in this case)
Errants
Vilmon + paladins
Good luck and have fun