My download speed is about 3.5MB/s and upload is 1.5MB/s. Can I build a barebones PC with a large hard drive to use to seed 24/7? Or do I need to build something faster? I'd like to do this for under $500 if possible.
Yeah that would work... just invest in a good quality (brandwise) power supply. It doesn't need to be high wattage, but something from Antec, Seasonic, Silverstone or Enermax is going to provide more stable voltages and last longer than some bargain psu from Diablotek, Logisys or Coolmax.