RE: Royal beechcraft 58-P baron
Loosenut ... you may get away with a 4S set up depending on the flying weight and 5S is for sure workable but try to keep her light. Capacity in the 3,300 range should be nice or maybe x2 2,500 packs in parallel for more duration. From the motor size they are recommending, I think go with a 4S set up.
For ESC look at a 60A one, I like the Turnigy PLUSH ones, they come with a built in UBEC (means you do not need a separate power for the RX etc) and will run the servos well. I used the 80A one on my Sebart Ms Wind 50 ... HS65 x4 and x2 standard servos.
Motor wise you want something that will spin something like a 10x8 in the 12K range. You have plenty to choose from. DualSky (4240 series), Turnigy, Hacker, Hyperion, AXI (2820 series) and more. This is just a baseline guide for you to work with. Maybe best to build her and find out the weight then and work from there. Remember to look into the following when building:
battery pack placement (electric motors are much lighter than glow engines, CG issues, use the battery to balance it out)
access to battery packs
cooling for ESC
ESC placement