Hello there! Welcome to TDC and glad you are getting into Dark Eldar. I just started a year ago myself and I did a ton of research to get started but I made my mistakes of course along the way of getting units. I would suggest you give this a read through:https://www.thedarkcity.net/t4935-dark-eldar-unit-guide-plastikente
That way you may be able to glean what kind of army you may want to lean towards. There are lots of options with how to build a list. Something you may want to consider is how many points you want to collect up to initially. What would you play with your friend in an ideal situation? From here its easier to start looking at the units you like (whether it be for looks, gameplay, or whatever) and determine a nice fun army. The link I gave you looks at the strengths and weaknesses of every unit in the book. Its not the end all be all of pros and cons though and you can certainly try your own combinations and tactics!
Based on what you got (I myself started with 3 battleforce boxes and a handful of other units). I would say it may be nice to get another Raider so that both your troops are in boats. Another good option would be an incubi squad in a venom, or a ravager. I think these would be good places to start, also another troop choice is not a bad thing at all. The question you want to answer is what kind of troops do you want to pursue. I think that you will find most people here like warriors currently, though wyches certainly do have their benefits!
Please ask questions! DE are difficult and everyone here is here to help as we can.
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