I guess that would depend on what your MAIN goal is of goals? Collection? ELO Prowess?
At this point, you have majority Nightmare, so you COULD start there.
After all, Nightmare is VERY competetive in all formats. Especially DMs.
Staple cards to look for there are Kolos, Nistarok, Oshitsune, Ghumbo... But thats a bit off your budget.
Assuming you don't have the means to buy more packs, just as Population 0 says, maybe buy more (cheap) nightmares so your bonus will be activated for your cards.
Cards like Sargh (~230), Orlok (~240), Eadh (~640), Timmy (~690), Dieter (~700), Estalt (~850), Mawpin (~116 would be a good start for a Nightmare deck. May I suggest getting Orlok and Estalt as they are a cheap way for a fury 2HKO.
Then again you have Aylen and Sung Tsu which are excellent 5* cards for their clan... But yeah, go Nightmare! (Remember, they are competeive in all formats. SO why not invest in something that you want to end up with anyway? Plus, they are nice to have on hand if you wanted to switch formats. Hope this helps