i looked for this cabinet http://zebronics.com/productDetails.php?id=13 it's only 15$ its has plenty of inbuilt fan but it'a a top psu mounting cabinet and there is no dust filters since it's a cheap case is that a major problem..
Top-mount PSUs are not a problem, it simply causes the PSU to run a little hotter which may reduce its lifespan, particularly for low-end PSUs and high loads.
As far as filters go, most cases are so ridiculous exhaust-heavy that it is a wonder why they even bother since the negative pressure people will generate by populating all fan mounts will suck dust through every other opening. You need 2-3 intake fans at medium speed to 'feed' an exhaust fan on low speed but most cases have twice as many exhaust fans as they do intakes, not counting PSU and GPU fans with their own exhaust which also contribute to negative pressure.
Negative pressure also makes all exhaust fans have to work harder to push air out and for GPUs where the shroud is not mounted flush with the exhaust bracket, it will also suck GPU exhaust back into the case.
I personally find it ironic how enthusiast cases seem to be designed to make things worse by having such a massive imbalance.
Top mounted should be fine, If your PSU is decent then I wouldnt worry about it, as long as the PSU can get some air. Normally them Corsair GX series psu's runs vary cool, but I can't say for any other band.