I am looking forward to rated play when I ding 30, but will only have a full AP rune set. I am wondering which champions I need in my repitoire.

So far I have:
Ryze
Heimer
Annie
Sona
Janna

Who else should I buy to round myself out?

Anivia is a very popular AP champion. She can deal a great amount of damage with little AP items in the early game and she's able to grow very strong after early game--she's great at taking mid lane.

Sona and janna fill a different role from the rest of the champions you mentioned. These two are focused on support (healing, shielding, etc) NOT damage like the rest of them are.

Anyway, some of the best AP carries are:
malzahar (probably best scaling)
anivia (most utility with wall, great burst)
kassadin (super mobile, good assassin)
vlad (very strong aoe damage with strong laning phase)
sion (fantastic ratios, his hard cc is really really good).

i would not recommend going into ranked play only knowing ONE kind of champion though. You should at least know tank and jungler roles (everyone wants to AD carry, never find lack of those).

ya, I know that they are supports- but I should be able to play support or AP with my runes.

Thanks for the lists, ill look into those.

Also- to your advice to learn more than one type, Ive done a few junglers/tanks/AD carries and feel very good with them (SC2 micro gogogo), but should I be sticking to AP since thats what I'll have a full runeset for? I was planning on joining a ranked game and announcing "only have AP runes" and avoiding other heroes, or should I be more flexible?

I guess my overall question is whats the best way to start ranked with only AP runes?

You have some pretty solid champs already. As should always be stated in these threads, don't buy a champion off of a reccomendation alone. Try them out first. Lots of people will tell you Anivia is amazing, and she's great, but I suck so horribly with her that I would never play her in ranked. I'd rather have a great Ryze player than an average Anivia any day.

also as was stated above, to truly shine in ranked, you have to be good at all facets of the game, otherwise if you have last pick and AP carries are covered by your team, and no one will switch, you have just severely gimped your team.

I wouldn't advise racing into ranked as soon as you hit 30. There is no need to rush at all. It might not be obvious but there are a lot of subtleties to the game that you still learn post level 30. The fact that you only have a rune page for one type of champ is reason alone to not play ranked yet as building a balanced comp is the most crucial skill to have.

I wouldn't enter Ranked until you have rune pages for AD, AP and Jungle. Are affluent in at least 2 jungle champs, and have mastered at least 2 AP and AD champs and hopefully have a few tanks you're good with.

Also you should fully understand lane mechanics, last hitting, counter-building, counter-picking, map awareness, ward placement, counter-jungling, lane ganking, minion lane pushing, etc. If any of these terms bring any confusion, don't enter ranked yet.

Just my advice.

Anivia, Swain, Malzahar, Sion or Annie
solo top lane, just keep farming the lane and letting your jungler gank so you get kills/free farm.
And remember to ward so you don't get ganked

Malphite. He's not an ''AP champ'' since building ap items is not the best way to build him imo, but his damage is magical so he uses caster's runes. Apparently, a standard caster runeset is very good on him. He's also very fun to play imo plus he's free for the week. You should give him a try.

I'm about to start ranked as well and AP champs are my forte, right now the choices I have at my disposal include a variety for counter picking, I have assassins (LeBlanc, Kassadin), tanky mages (Vlad, Swain), general mages (Malzahar, Annie), support/utility (Janna, TF). However, I also have an AD page that doubles as a jungling rune page - and I have some AD picks that I play well ~just~ in case my team is already too AP stacked. But at the same time I'll be duo queueing with a buddy who often plays tanks. P:

I'd honestly suggest to wait til you get more AP champs and/or build an AD page for more variety. :3