as far as carbon scrubbers, the ebay one looks good. i use one from the exact same ebay seller, just one size up for my small closet grow (
http://cgi.ebay.com/ws/eBayISAPI.dll...m=160229575444). the extra $$ for the next size up might be worth it if you have any concerns over smell leakage with neighbors and all.
don't know much about build-it-yourself ballast kits, but that kit should work well. you'll need a HPS setup for flowering, which will be a whole other bulb and ballast setup. if you want a suggestion, return your MH bulb to lowe's, and then buy a HPS ballast with a HPS bulb and MH conversion bulb. several ebay sellers offer these setups as one whole package. this way you only need one ballast for both bulbs.