If your tight on your budget you could go flowering with a MH bulb as latewood said without any problem (I strongly suggest a warm bulb or any bulb with enhanced red spectrum if you plan on flowering with it). However, if you can afford a conversion bulb (You can buy a HPS conversion bulb that fits your MH ballast) or HPS ballast then id suggest you buy both MH for Vegetative Cycle & HPS for Flowering Cycle

Have a nice Grow!