Okay - lots of questions, lots of answers:
FIRST: You can't race or often even be on tracks if you have pegs on your bike due to insurance and safety concerns! If you aren't planning on competitively racing, then build your bike up for strength. If you are actually going to go out and competitively race, then any street you do on your bike will likely damage your bike and make it slower on the track. Freestyle is incredibly abusive on bikes and good freestyle bikes build around strength, not weight. Racing is all about cutting the weight appropriately.
1. 36H and 48H refers to the number of holes in the rim and the number of spokes on the rim. Racers usually use 36H or less rims because of the weight savings, while most (most!) freestyle riders use 48H rims because of the added strength from the rim.
2. Flanges are the parts that flare outwards on either side of the hub. The generic term 'flange' from www.m-w.com
: "1 : a rib or rim for strength, for guiding, or for attachment to another object <a flange on a pipe> <a flange on a wheel>
2 : a projecting edge of cloth used for decoration on clothing <a jacket with flange shoulders>" Some flanges are bigger than others the bigger flanges are called high flange, the smaller flanges are called low flange. You add weight as the flange size increases.
3. Ads in magazines are designed to sell you product regardless of the quality of the product. Dans will not advise you of product quality, they will only sell it to you if they have it. So ALWAYS ask prior to purchasing a product if you are unsure of the quality of it.
4. If you are going to ride street then get high pressure tires. You can always take air out of the tire, but high pressure tires have added material (weight!) that allows them to hold better to the rim and to not pop when you put upwards of 100PSI in them. I personall run about 105PSI in my tires with no problems at all... but I ride flatland.