Depends on how you good you are, if you have some greenstuff knowledge, it's the best way to make your own harlequins from all kind of models. The empire has a lot of nice bitz, though I don't find they all suit the eldar aethetic, some greenstuff solves the problem.
I'm sorry to say greenstuff is a must, even if you use just a little, just kitbashing various kits together won't do it.
The wyches are the best for a starter pose, scourges mostly the leggs, but I would stick with wyches, scourges are more expensive, maybe just 1 or 2 boxes for the bitz. Hellions for the bitz, but I'd say it's even worse then the scourges, the leggs aren't usuable for harlequins, they're too static.
Weirdly enough a lot of old dark eldar kits have a lot of dynamic poses for harlequins, the old Lelith and Kruellagh has been used in a lot of harlequin conversions.
The warrior and wyches coming with the raider and the venom also have very good posses for these type of conversions, buying the full kit would be a waste, though for the venom, that is a transport used by harlequins, a different version, but still. You can find them on ebay or trade/buy from player, the raider warriors aren't all used by most players.
sry don't have a better picture, I always think of this squad when people mention converting harlequins for some reason.
BTW there's no such thing as dark eldar harlequins, you just say Harlequins. They're not craftworld (though they tech is almost 90% craftworld, 10% of their own stuff), they're certainly not dark eldar, they use 0 dark eldar tech.
One of the logs on this forum, it's mostly empire harlequin based, the guy is really talented, though it doesn't fully follow the eldar aethetic, it still is a very cool army. https://www.thedarkcity.net/t6097-harlequin-carnival-la-danse-macabre
I approached this differently, I bought all the oldschool harlequins dating from the rogue trader days, some are very ok, others need a lot of work, all the models have the correct bodies and poses to start with, weapons and a lil extra may need some extra work.
I also got the currenty harlequins to have all te harlequins released by GW. I actually changed some of the old ones, put them on a stone on a base, changed the pose somewhat ( this is when you love metal minis), put some extra cloth pieces and they look very similar to the new style harlequins.
Another important aspect on harlequins I see a lot of people ignore on the net, but it truly matters: the harlequins kiss. If you're converting for a cool looking army, for a painting contest it's okay to leave them with swords and other beautiful looking weapons, however if you want a squad good on a battlefield, you'll need the kisses. The kisses make the squad excellent, without it they're simply not as good. They're more expensive, but 100% worth it. Most people do keep a power on the troupe leader for a 100% garantee of no 3+ saves, though it used to be better and the low strength can be a problem.
I have some pictures about this on my warseer blog