It's hard to go wrong with Elves in general. It's hard to screw up a list and while it Elves will not be winning tourneys (short of maybe huge Spireguard blob spam), the lists are fun and generally competitive without being overpowered.
I actually run an Eldritch Council list with is basically two Dragons (Archmage and Drakeseer) with a Regular Archmage and Loremaster, tons of Swordmasters and some Sisters as allies. It's not that competitive (I would say I win as many matches as I lose with it), but good god it's fun to play.
For Bolt Throwers, I would check E-bay. The Skycutters box also comes with a smaller Bolt Thrower as well:
https://www.games-workshop.com/en-GB/Skycutters It's expensive but the Skycutter is an awesome flying chariot to slam into the flanks units to support a charge that can also do work at range with the bows. if you opt to take Spireguard as passengers instead of the Bolt Thrower. Then you can just use the mini Bolt Thrower as a regular Bolt Thrower if you like. Honestly I would consider a box or two as they are very good Chariots, and the Skywarden option is basically mandatory anyway if you have a lot of Swifthawk units in your army (take the Pennant and the Battalion from the Spire of Dawn book to give your Swordmasters 2+ to wound!)
EDIT: I would also magnetise the Hawk as well. It means you will have the option to use them as Great Eagles if you want as
GW doesn't sell those anymore.
EDIT 2: Wait, nevermind, they sell Eagles as part of the
LoTR/Hobbit range in plastic so maybe look into them too if the list is for fun!
2000 Stormcast Eternals
2000 Aelfs
2500 Legions of Nagash
2500 Ultramarines 2nd Company 
