generally mate i think the consensus on here would be to not go cheap when it comes to buying coilovers!
if you are doing it on a budget its probably best to go for a good set of springs e.g. Eibach or H&R which depending on what type will cost you anywhere between £150-£250
a decent set of coilovers can cost you anywhere between £650 and £1300+
E92 - 325D M Sport