First of all, the 15 deg. skew. Yes, the angle should be 15 deg, but in a different plane than what you have drawn in your drawing. See this image for an explanation.
http://www.anotherpower.com/gallery/decogging/figure_8
Notice that Beta should be 15 deg in your case, but, that Delta most likely isn't 15 deg.
Secondly, before continuing. I notice that your magnets are drawn pretty close together. Also notice that at the bottom, they will be closer together (and likely overlap...) because of the smaller radius of the rotor at that point.
What I'm trying to say: the way you have drawn the 'outlay' (is that proper english?) the magnets MAY not fit, depending on how you will build the rotor.
Another builder of conversions, 'Behoof', found this out the hard way...
Whether this is the case for you depends on how you will actually manufacture the rotor. Hopefully you can tell; will you make a new solid steel core, with an aluminium sleeve ? Or will you mount the magnets on the turned town old rotor (I understand that's your plan) and then cast in epoxy ? Please answer.
If you plan to place the magnets on the turned down old rotor, you should be calculating the skew angle using the TURNED DOWN diameter of the rotor, NOT using the final outside diameter of the rotor (approximately the diameter of the stator). Hope this makes some sense.
Anyway, try to read and understand page 6-13 and p.27-28 of the decogging tutorial:
http://www.otherpower.com/images/scimages/3538/decogging_tutorial_V1.pdf
If the math bothers you, give all the relevant dimensions as shown on p.28-top of the tutorial: heart-distance between the extreme magnets 'L', diameter D of the rotor-surface ON WHICH THE MARKING WILL BE MADE (important; and it depends on how you intend to manufacture the rotor) and the amount of magnets per line (7, by the looks in your above drawing). Give me that info and I can calculate your correct skewing angle for you.
"I am getting 3 types of disc magnets here. 13 X 10 mm , 10 X 10 mm and 20 X 6 mm disc magnets. Which type will be more suitable?"
The most suitable magnet is the one which you can stuff the most of into the rotor. The more magnetic volume and area, the better. You'll have to make some sketches for yourself and see how much magnet you can put in. My gut-feeling is that 13x10 mm would be a nice size, though a bit high (7 mm instead of 10 mm would have been nicer); the 20x6 mm ones have a pretty large diameter so may need a lot of airgap to clear the stator. To give you an idea, my 130W genny used 12x6 mm magnets and the 3 hp used 20x10 mm ones. I think I'd choose your 13x10 mm ones but again, make some sketches to see how much magnet you can stuff in there, for the various sizes.
"But still I wanted to know if 'X' HP of motor produce 'X' HP of power if converted nicely."
I have converted a 130 W motor and did some measurements w.r.t. power output (battery charging) for various system voltages (12-24-48V). With its original winding (3 phase 400 Vac), the generator was able to put out 130 W in a 48V system; for 12V and 24V systems it was lower. You can find the measured data here:
http://www.anotherpower.com/gallery/album49/130W_genny_table_output?full=1
http://www.anotherpower.com/gallery/album49/130W_genny_graph_output_001?full=1
Notice that the maximum output was at 1000 RPM, which would be pretty fast for a windgenerator.
The above graphs are only an indication and your results will likely be different, they depend on how much magnet you put in there, the airgap, the grade of the magnets, the position of Mercury w.r.t. Venus, etc.
So, to conclude: your skew angle of 15 deg as you've drawn it is incorrect (it should be 15 deg, but in a different plane). Depending on how you intend to build the rotor (alu sleeve, cast in epoxy), the outlay you've drawn may or may not be correct as the magnets will overlap/touch eachother.
I hope the above wasn't too rambling. If some things aren't clear, let me know.
Peter.