I'd recommend:
STR: Leoric, Nessaj
AGI: Mortred, Riki
INT: Dark Seer
after Skeleton King, one of the best choices to continue with is probably Chaos Knight; you'll learn even better how to conserve mana than with Leoric, and Nessaj is quite hard to kill thanks to having highest base movespeed in the game, being able to Blink Strike to allied units, and in worst case scenario even Phantasm can be used when being ganked.
Nessaj is a really good hero to learn how things go on in mid-game with, since he's a great ganker, and you will suddenly notice that you're the one who's leading ganks. He is also a really good carry. His cores are actually quite cheap, Ring Of Basilius, 2x Bracers, Helm Of Domi, Power Treads, and now I'd say Armlet but that is hard to use for new players, so Battle Fury works just as well. After that, Cuirass, Satanic, Heart, BFly, MKB, BKB or more Battle Furies are all options. When you get the hang of him, you can even try to do a Radiance rush during the first 30 mins.
After Nessaj, it's easy to go for AGI blinker carries like Mortred and Riki, who have to be played quite similarly. BFury, PT, HotD, BKB, another BFury, MKB, Satanic works for Mort, and Midas, PT, Diffusal/Deso, Bfly, Manta, Vladi (then sell Midas for Buriza) works for Riki. Get Ring Of Regen, Quelling Blade and Poor Man's Shield early on for them to have more staying power and to farm easier.
As for INT, Dark Seer (along with Ogre) has the highest hp stat among int heroes, and Surge makes him and his allies extremely hard to kill without disables/silences. Not hard to use either, and provides some of the best support in the game. Vanguard, Orchid/Guinsoo, Boots Of Travel, Shiva/Cuirass, Khadgar's Pipe, Heart, Radiance, Skadi are all great options for him (and so is Necromonicon but I don't recommend that for newbies).
Also Kunkka (the loadscreen maker for dota) said the new loadscreen for 6.65 is already finished, so I suppose 6.65 is just around the corner now!