Buy reinforcements if you feel you need them, but really you won't. You'll find them easy enough and by now you know what to do with them. There's a mercenary camp, goblin merchant, and various fountains on this map. You don't want them to know you're out there until you've rid yourself of Sylvanas and her pesky blue elves in the northwest corner of the map. But whatever you do, DON'T CROSS the southernmost river into the lower third of the map (the city of Silvermoon proper) yet. Once you have enough guys together, you can send a pack out with Arthas to start exploring the map south and southwest of your base.
In this mission it didn't matter too much, so a mixed force is the best default - cue up to build ghoul, crypty, garg, ghoul, crypty, garg.
Generally I just click on a crypt, then fill up all the building cue slots with whatever units I need/afford, making a snap determination based on what kind of enemy I'm fighting and how much money I have. Build up all the crap that you know you need - Necromancer's house, more Spirit Towers, perhaps a second Crypt, and lots and lots more troops.
