I suggest starting around the Very Powerful (mystery men) 275 point to Low-Powered (youthful superheroes) 300 point level to make Alpha and Beta level villains a credible threat. Player teamwork and skillful use of game mechanics make straight character point comparisons an unreliable mechanism for determining whether something is a challenge. If you start smaller, you can always be generous with experience points to grow quickly if you don't like the scale.
Having played Champions from 400 point characters to almost 800 point world-shapers, I have found that more points does not always lead to more fun. While I could do more stuff with 800 points, I didn't have twice the fun I had when I started off with 400 points. Since I know you've played City of heroes, ask yourself was it fun for a level 50 toon to smash those level 1 to 10 Skulls? Probably not. You enjoy struggling against a comparable threat. You want to work your way up to those Gunslinger Malta agents.
At the moment, I'm playing in a low powered 150 point supernatural hunter campaign and loving how well HERO scales down: all the flavor for half the calories. If you haven't already, I highly recommend you check out Killer Shrike's website for HERO resources.
You didn't indicate how frequently the group will play or how you typically award Xp. For a long term campaign that meets weekly, 300 or 400 starting CP is fine. They will become formidable within the first 2 years. If you're gaming monthly, perhaps 400 or 500 starting CP is better for that pace.
