Well usually, friendly players are not strict WYSIWYG lawyers. So, you'd be just fine NOT having for example a standard and a champion in your dark rider units....
With regards to "next buy", dark elves offer a wide range of models and specialised roles suitable to many different playstyles. Do not, however, attempt to shoot against your enemies as a source of victory: in spite of darkshards being "oh so famous for their rain of death", in reality, dark elves lose the shooting contest with so many other races its not even funny
a little shooting to take care of flak and pressure monsters is fine, but no more than that.
Generally speaking, the power units of each respective category are:
Lords: Lvl 4 (duh), ideally mounted, ideally on a dark steed. Variations include having her on foot in a babysitter unit, e.g. spears since they are cheap, or corsairs since they are durable to shooting. Additionally, mounted dreadlord, either on a steed (popular build: full mundane armor, dawnstone, giant blade, other tricksters' shard for 1+ rerollable armor save, S7 attacks and enemies must reroll wards) or on a pegasus (popular build: cloak of twilight + dawnstone).
Heroes: Mounted masters. Here, the pegasus far outweighs any other option since you get 3 wounds and T4 on an elf. You also get to fly, and a stomp attack. Stick regular 4+ ward saves on them, and give your BSB a charmed shield as well.
Core: Dark riders for board control, doubleflee maneuvers, hunting enemy soft targets and the like. Witch elves either in big blocks for serious combat punch or smaller units for skirmishing. Remaining core options are generally "not worth it", although corsairs and spears/swords can be fine as a "hang back and dont get into trouble" unit.
Special: Bolt throwers are the obvious include here. They shoot very efficiently. A unit of shades is also a strong option.
Rare: Warlocks warlocks warlocks! Stick your lvl 4 on a steed with mr(2) in a unit of warlocks, and bam. 2+ ward save vs magic. And a highly mobile caster to boot. Popular options are death, death and death. A mobile purple sun as well as being in range of the relatively short death lore sniper spells is huge, and what lore of death does not have in direct umph, the warlocks supply with their own built-in doombolt. Buy them, use them, abuse them! Just keep them out of sight of ballistic skill shooting... A 4+ ward is equally good vs a cannonball and a skink javelin, once you're wounded!