Can I go for PG in medical or IAS preparation after completing MBBS? I am confused.

Yes definitely you can go for UPSC after completing MBBS. Infact you can appear UPSC during your internship also. But remember both PG and UPSC are equally difficult to crack.

What is within your reach and what you truly want? Remember that the hard part of IAS is interview, would you be able to clear that? And, also since you have studied MBBS, you will have to study GS and History for IAS from beginning. On the other hand, PG course will be based on what you studied in

There are IAS officers who had completed MBBS. It is an additional qualification to serve the society.