You recommended another Berserker or Assassin, but I'd rather have unique classes, because I'm weird like that. Would a Ranger be a reasonable substitute? Otherwise I might just try and get a Pirate...
I'd pick a Templar personally instead of a Ranger, though there are some very good ranged weapons in the game a bit later on. Having a Fighter-Cleric (Templar) can also come in handy at times, though many of the other spellcasters will also learn the healing spells.
Are there any skills that are so niche that they can be ignored entirely for a first, non-completist attempt?
You can safely ignore these skills:
Evasion The ability to make a safe strategic retreat without injury during combat.
Throwing The skill of throwing objects so as to strike a target.
Mephistics The art of applying poison to weapons so as to increase their effectiveness.
Nature Lore The art of identifying and mixing ingredients in plants, powders and reagents.
Metallurgy The ability to forge powerful weapons and to construct them from components (Yeah, I don't think it's really necessary since the effects seem to be temporary)
Nice to have, but not really necessary:
Stealth The ability to hide oneself amidst shadows or behind terrain features.
Robbery The larcenous art of relieving strangers of their valuables without discovery.
Berserking A deadly art involving a special method of attack with weapons in combat.
Barter The ability to bargain with merchants and get the fairest price in commerce.
Incantation The ability to recite spells written on scrolls and cast the successfully.
Have one of the guys study
Invocation to use the magical items you come across, but also quite optional really.
Needed only somewhat later in the game:
Ancient History Knowledge concerning the distant past of customs, peoples, locations and events.
Climbing The physical skill of climbing wals, ascending cliffs and dropping into pits.
Important for spellcasters:
Scribe Degree of literacy with written works and ability to accurately copy spellbooks.
The Magic Skill each spellcaster is using is important too - and the guys that will scribe Arcanum spellbooks should later put some points there as well.
Important for melee:
Lethal Blow The knowledge of critical body areas and targets needed to strike a fatal blow (NOTE: Needs weapons which have *Lethal* in their stats when you assay them.)
Specialize in one or two weapons skills and the Shield skill can come in handy too.
Music instruments can be quite good (unlimited spells basically) for attacking groups of enemies. The Music skill also goes up through use, though relatively slowly.