If you're looking to sell it in the next 6 months for definite then just get the best quality stainless steel budget exhaust you can, ...no point spending more money than you need to on something you're really only going to benefit from for 6 months.
I'd say just get a Scorpion or Cobra as both are around £400 odd so great deals. See Tarmac Sportz or the Cougar Store (both in the trader section on here).