I don't think you can take Extra Invocation yet. IIRC, the extra must be 1 level less than what you can use, and since there's nothing below "least", you can't take it until you can invoke "lesser".
Some other things you might want to check out:
Hellfire Warlock PrC (Fiendish Codex II, p89) Using their special power to increase damage means you take a point of CON damage. This is often dealt with by taking a level of Binder (Tome of Magic) and binding Naberus (sp?), which lets you heal 1 point of ability damage/round.
Chasuble of Fell Power (CAr-148) - adds +1d6 to your EB for 8000gp, or +2d6 for 18,000gp.
Mortalbane feat (BOVD-49) - your EB is a SLA, so this lets you add +2d6 damage 5/day.
Precise shot - lets you fire into melee. Probably won't need it, since you're doing touch attacks, so your to-hit number should be low anyway. Still, 20% can make a big difference.
Binder Class - if you're taking it for the Hellfire class anyway, get it early. Bind the guy who gives you "ricoshet shot", which basically lets you attack 2 different targets with the same shot.
Bind Vestige Feat (Tome of Magic-72) – lets you bind a vestige like Binder 1, but only gain 1 of the powers – but requires only a feat, not a whole level.
Rod of Eldritch Power (CM-127) - contains an invocation that can be used 5/day. If you're in luck, your DM will let you pay 1/5th the price for 1/day, then use it for the powers that last 24 hours so you only need it 1/day.
Least – 2000gp; Lesser 8000gp, Greater 18,000gp; Dark 32,000gp
Horizon Googles (CM, p133) – gain Far Shot feat, also increases range of spells and SLA that require ranged attack rolls by +50%; 8000gp (not needed if you take Eldritch Spear and so have a range of 250')
Codex Advocare (Expedition to Castle Ravenloft) – book that grants a warlock 1 extra least invocation as long as he keeps the book on him. Can only have 1 book at a time. 20,000gp.
Good luck!