This was discussed in detail in a couple of topics, but I haven't been able to locate the threads yet, so this may sound familiar.
The level of the NPC isn't as relevant as the number of NPCs vs. number of PCs. In one thread, a GM sent a pack of 10 level 1 rats at two PCs and the PCs lost. While the rats had low HP and DB, the PCs were still faced with (at best) 5 attacks per round with the ability to parry vs. 2 attacks. That left 3 more attacks with no DB (other than magical) at each PC.
The important question is the number of PCs in the party, as Majyk mentions. What are the PCs' OBs and DBs and HP? I routinely send 1-2 level 4-5 NPCs against my group of level 1 PCs. I'll have 4 - 6 PCs per gaming session. This gives the NPC some lasting power, but enough OB to force one PC to parry. The other players position themselves for flank and back attacks to gain bonuses to their decent OBs. The NPC level allows for decent spells if I send a spell user, or decent HP for a fighter-type to absorb some blows from several attacks.
If the party is facing a boss or mini boss, I make spell casters at least level 7 so they can get off a spell per round and I'll give them a couple of underlings as cannon fodder. If it's a fighter-type, level 7 with a level 4-5 underling will give very nice OBs and force at least two players to parry and it will make positioning all the more important.
If you have a party of 7 PCs, two level 5 Orcs/Thugs should be fine. Three PCs per orc while the spell caster stays back from melee to prep/cast spells. At least one PC will get a Flank or Back attack bonus. Or send 3-4 level 2 orcs/thugs. These numbers only work if your players work well together. If everyone goes off on their own trying to do their own thing, they'll be toast.
Take a look at the lowest and highest melee/ranged OB from the players. Get an idea of the level of damage they can dish out and give the NPCs an OB in the range of the 2nd and 3rd highest PC OB.
If you are sending lower level NPCs, give them stats very similar to the PCs' stats. The PCs are a good indication of what level 1 NPCs would look like.
For the spell users, give them a chance to see that a battle is about to happen so they can start prepping their spells before combat starts and they can cast a spell in round 1 of combat.
Give them smaller situations leading up to your essential scenes. A random encounter with a wild animal is a good object lesson. Give a low OB and higher DB and decent HP for the animal. The DB is too high for a single PC to hit and it won't dish out a lot of damage to the party, but when a couple of members move to flank the target, they'll see that no DB +15 Flank attack yields nice hits. Hopefully they'll carry that over to the main combat scenes you have planned.
