ur mixing is where things goes wrong.

may i ask why are u going for such high end microphones? if u're getting a condenser sennheiser mic, thats definitely going to be very expensive, and with ur soundcard, theres totally no point at all. then there'll be an issue with phantom power to power up ur mics(if u go for good recording vocal mics, which i would naturally go for a condenser).

that budget u cant even get a shure sm58 dynamic mic. lol. well u can try the local made taming sari mic. i heard samples and its good. contact bassraptor/sujesh if ure interested. remember u would need to spend another extra rm400 roughly for a DI box with phantom power.

amp is totally another thing. condensers need to be powered up so thats what phatom power is for. your case just get a dynamic mic. normal karaoke mics are normally dynamic mics.

sm58 is a dynamic mic. its above the rm400 range. its the industry standard dynamic mic for normal live usage. nothing special. US list price is USD$188 but online sites selling it at around USD$100

why i suggested the DI with phantom power is coz its within reaching distance of his budget (which is around rm400 for a dbx one, whereas behringer mic100/200 will be less than that). going for a proper phantom power unit would be silly as he doesnt even have money to get 1 proper mic.

as for mixing, its 99% EQ, levels and compressors. as easy as it sounds and it is to learn, it takes years to master. mastering has alot of explaining to do (and also the fact im not so good in mastering myself). as ur backing tracks are already done, i assume theyre already mastered. so i guess u can get away(sort of) with mastering ur vocals if u mix it in just right.