The thump comes from a ported box. You'll have to get a box tuned correctly for your sub or you'll destroy it. The information should be on the manufacturer's web site - if not call them and see what they suggest. The box is the biggest bang for your buck.
Get the largest diameter sub you can...
No, not OK. The ohms represent the resistance (in the case of speakers, its called impeadence). The higher the value, the harder it is for current to flow through it. Your head is designed to drive speakers that have 4 ohms of impeadence. Your cab has 8 ohms of impeadence. Your head will have to...