Why not use a longbow?
The interesting thing about the EK is that, because it is a generalist of sorts, you have to "re-specialize" it a bit to get power out of it. IMHO,the D&D model of the four-person party simply does not reward a generalist sufficiently. Thus, you should think about what exactly you want your EK to do. Being the party spellcaster is clearly a no-go, because then you should go straight wizard or cleric. So it's gotta be something else.
One of the nice things is that the EK can fuel other builds (bladesinger, spellsword) to make them work better than just taking the PrC would. For example:
1) The Bladesinger. Ftr2/Wiz4//Spellsword 1/EK 8/BS 5. Concentrate on mobility feats (including Quick Draw, for neat wand/attack combos). You will have a caster level of 15 and BAB +18 at the end of this progression. In mid-levels, act as a tactical support (mobility) fighter and cast exclusively utility spells. At the high end, you can buff yourself to the roof, utilize polymorph in conjunction with fly and ability buffs, and hammer your foes into the ground.
2) The spellsword. Wiz4/Ftr2/Spellsword 5/EK 9. This build uses heavy (mithril) armor, Extend Spell, Persistent Spell, and LOTS of buffs. Caster level 15, Spell failure mitigation 20%, BAB +18. This build is easily usable as a tank.
3) Pal2/Sor6/EK 10/something else 2. 9th-level arcane spells and Charisma bonus to saves. (DanMcS, Pal2 is a MUST. Having Cha bonus help out with spellcasting AND saves is awesome.) This is a pure combat mage build; you have the advantage of paladin levels to provide you with a few extra hp and that save bonus!