Good luck with that! Nightmare is one of the more expensive clans and Uppers isn't much better.
what kind of budget are you looking at?
Eadh, Timmy, Dieter, and Hel are cards that you are likely to find in both the upper and middle ends of Nightmare decks of those currently under 2k, but those 4 by themselves isn't enough. Glorg, Nistarok, and Sheitane round out the list of the non-super big ones with an honorable mention to Ielena. Azel is still too new, if you are on a budget, wait a few weeks. If you are in a pinch, add Estalt due to the large damage, but know that he is going to have problems.
Frankie Hi should be on your list for Uppers, and Stanford can do enough damage for a 3* to make him a threat compared to his cost on the market. Rubie is nice for the SoA, but her power is a little low. You should probably get her, but it should be a lower priority because I feel that playing her will be too obvious if you don't have good cards to support her. Beetenka, Jeeves, Samantha and Mickey T round out the list with honorable mentions to Sydney, Janine, and Colin for not being totally useless. Gina Glitt could be used, but the cards I list don't deal enough damage to make it worth while.
I think that getting Dorian if ELO or Jackie and Zatman should be a higher priority than Kolos or Kenny if you stick with this group.