With a 5.5 lb flying weight you should be good with something like the Cobra motor I mentioned in my last post. I did not look through all of the options, but look for something that will give you 550 - 750 watts with a 4S (14.8v) battery and something close the the 12x6 prop.
The other thing to think about is the all up weight of the motor, battery, ESC as compared to the glow setup. At 5.5 lbs, I am guessing that this is the 4* 40, correct?
Assuming so, a typical glow 40-46 sized engine would be somewhere between 16-18oz, plus another 3-4oz for a fuel tank, throttle servo, throttle linkage, etc. The Cobra motor is 7.6oz, a 3000mAh 4S battery is about 10oz, and another 2oz for the ESC and you are at just under 20oz total for the power system. So this compares pretty well to the glow setup.
If you have not flown electric before, do you have a charger for the batteries?
There are all sorts of preferences out there for batteries, connectors, chargers, etc. Truth is that they are all fairly comparable, but once you settle on a specific solution, then it is usually best to stick with it. For example, I prefer to use the EC3 style battery connectors. If I always stick with these connectors, I never have to worry about adapters. Other guys I know prefer "T" connectors, while others prefer "XT" style. They all work fine and it's mostly a matter of preference (unless you are running a real high power set-up drawing 100+ amps).
There are other considerations too. The golden rule is to never run you batteries below 3.7v per cell. Doing so will likely damage the battery pack. I try to limit my flight times to never run my batteries below 30% charge capacity. This will keep me above the 3.7v limit. Also, NEVER overcharge a battery pack as that can lead to a fire.
I know it all seems daunting at first, but once you get past some of the jargon it really is simple stuff. Do some searching here and at other sites and talk to others at your field and you will get the hang of it quickly.
Please don't hesitate to contact me directly with any specific questions you might have. I will send you a note via e-mail so you can have my contact info.
Al