Hi erg,
Here's just a little more advice. Download motocalc http://www.motocalc.com. The first month of use is free.

This tool has already saved me tons of money, I just wish I had used it when first doing my Guillows P-40 (28") rubber to electric conversion. The program is a little overwhelming at first, but when doing this type of conversion you can easily spend several hundred dollars before you figure out that you bought the wrong stuff.

To give you some weight specs, my P-40 is weighing in at just under 10 ounces with servos (GWS Naro), ESC (Great Planes C-10), and receiver (GWS Pico 4 Channel) installed. This weight also includes the entire plane being sheeted in 1/32" balsa. With the motor and battery installed it weighs around 14.7 ounces with a 15 ounce wing loading. This is still fairly heavy, but not too much so.

A caveat to building the Guillows kit is that their balsa is very dense (ie: heavy). Another is that each parts sheet is of varying density which means that things like one wing half will weigh more than another. Just be aware of this and sand / build to compensate. Replace their parts with parts cut from less dense balsa where feasible.

For my plane I originally thought the Great Planes T-280 motor, which is a Mabuchi RC-280SA-2485 in disguise, would work. MotoCalc proved me way wrong.

After some measuring for the proper airfoil parameters etc and plugging these numbers into motocalc, I found that Graupner speed 300 6V (#3306) with a 4.5 to 1 gearbox would work nicely. (Don't use the 7.2 V version of that motor, it is completely different and won't provide enough thrust to get your model off the ground)

For electric power I'm using a 6 cell 650mAH NiMH battery pack. You might look into either lithium poly or lion pack which is much lighter for the same power. Cost from eTech or Kokam is comparable or even cheaper than the Nicad or NiMH packs. At the very least don't use Nicads, way too heavy.

Another motor possibility are the GWS EPS300 systems, although these seem to be disappearing from shelves. I get the feeling that GWS will stop making them soon. On the other hand, the GWS EDP-400 direct drives provide some nice power.

One last thing, at every point in the building process think about what you could do to make it lighter. With these small electrics weight is the greatest enemy. Use lightening holes, change the guillows supplied balsa with something less dense where possible. When balancing, instead of adding weight to one side, look into removing weight from the other, or even try to move the battery around to reset the center of gravity (CG).

I can't say this enough, get motocalc, run the numbers, then research, ask a bunch of questions, run the numbers again, then buy what you need.

Good luck!
Chris.