I would buy what your budget allows lol.
menthol and koolada ... if you like minty menthol I'd pick up some obviously menthol or the crystals and make your own depending on how much you really love menthol lol. I have 10mls pre mixed menthol flavoring that will last me a lifetime so one bottle of flavoring is fine but if I wre into menthol flavors I might just buy the crystals. also I would pick up some koolada at some point. I prefer koolada over menthol most times just because I dont like menthol. it just makes vapes "cold" im actually vaping some koolada now in a vape. it doesnt have a lasting effect like menthol would, the koolada will eventually fade away so I probably would not make huge batches with added koolada but it doesnt fade as fast as a lot of people say either.
EM... I use vary rarely but it can be useful at times to take the bite out of some vapes and blend them a bit but you will sacrifice in overall intensity of the vape . If I use it its because I am probably looking to try an achieve a texture rather than enhance the flavor of something. usually I can find something better . I cant remember exactly so dont quote me but I believe EM changes the way your tongue taste things. its a lot more than that but basically thats what it comes down to. not something I would vape a lot of.
AP... I dont care for the way it taste but a lot of tobacco recipes benefit from it and there are lots of magic tricks you can pull off with it if you invest some time experimenting with it. I do keep it around just because eventually it gets used now then.
vape wizard... have it but never tried it. so I dont know lol
smooth, .... have it and dont use it. I dont get any effect from it really but some do.
those are the main ones I guess. I know there are more but in a random ramble because I seen my name mentioned above thats what I got lol. the 3 you might use the most of are the first 3 , menthol , koolada, EM. I wouldnt say you "need" them tho. like amanda and huck I rarely use additives at all. there is this big myth that you need all these fancy additives to make great flavors. great flavors come from lots of practice, a lot of failures and asking a lot of questions

good luck on your DIY adventure ! you picked a great forum to learn DIY.
Oh and bull city is the bees freakin knees. the dude is super cool just send him an email to see when he might have something in stock again. its never long usually.