Is it difficult to port and polish the intake manifold?

It would be very easy to do it yourself very badly and make it flow worse than it did in the first place. Unless the manifold is really bad to begin with (which it won't be on a relatively modern Nissan) you're far more likely to make it worse than improve it unless you really know what you're doing. Really knowing what you're doing would ideally involve years of experience and many hours of testing on a flowbench. The gains from polishing the manifold on a mostly standard engine would be so tiny that you would probably never notice so I generally wouldn't bother. If your manifold happens to be really rough on the inside you could try polishing it but only to remove the smallest amount of metal possible.

just buy an MEVI or OOVI you could gain something like 20-30 whp so I hear.