Honestly, I consider the lurker to be of approximately appropriate strength for a level-1 summon. After all, it does have some measure of range and can travel over water. Anyway, I think the best way to judge the Lurker is against other spheres' level-1 summons. Here is the comparison of stats from the Dwigg's 1.4 Documentation:
Dwigg's 1.4 Level 1 Summons: (Relative to Lurker)
Pros: Fire Strike, Ignition, Fire Immune, +2 Atk, +1 Dmg, +2 Def, +4 Mov. No "Animal"
Cons: +10 Cost, -1 HP, No Range, No Swimming, No Water Concealment
-=Dire Boar (Earth)=-
Pros: Forestry, Vision I, Charge, +1 Dmg, +4 Def, +3 HP, +4 Mov
Cons: -1 Atk, -1 Res, No Range, No Swimming, No Water Concealment, No Poison Immunity, No Posion Strike
-=Zephyr Bird (Air)=-
Pros: Flying, Vision I, +8 Mov
Cons: +20 Cost, No Range, No Water Concealment, No Poison Strike, No Poison Immunity, -2 Dmg, -1 Def, -1 Res, -5 HP (!!)
-=Black Spider (Death)=-
Pros: Web, Wall Climbing, Night Vision, Cave Crawling, +1 Def
Cons: No Poison Immunity, No Swimming, No Water Concealment, No Range, -1 Atk, -1 Dmg, -2 Res, -2 HP, -2 Mov
Pros: Floating, Holy Immunity, Better Range, +3 Def, +8 Res, No "Animal", Healing at medal
Cons: +10 Cost, No Melee Attack, -3 HP, No Marks. 1 at level.
As far as comparing the Lurker to the Purger, on paper it looks ok. You were talking about how the Lurker does about as much damage with its strike as it does with its range... compare it with the Purger who has no melee capability at all! Healing on level may be a bit much, though. It does cost 10 more, which is significant if you are spamming them.
As far as roles go, I see the Lurker as a balance of mobility with its Swimming, range with its spit +marks 1 on level, melee with its decent strike+poison strike and stats, and stealth with its water concealment. It's a pretty darn flexible unit. Yes, the Zephyr Bird is better at mobility, the Purger is better at range, and the boar is better at melee. However, none of those can reciprocate against the wide variety of enemies that the Lurker can fight against.
You do make a good case for the Purger being nerfed in some way, though. I feel that Healing and Path of Life should be removed, personally. Healing because I feel that it may be too useful to give to anyone who has any life sphere on a level 1 summon, and Path of Life because I feel that only really strong units should have path of ____ because it adds to their mojo.