it use to be around 10lac and than it jumped up to 13lac but due to price hike and expensive parts in normal condition will cost you between 13-15lac. However, good conditioned and well maintained vehicle would always be over 16/17lac.
No its not necessary to change engine if the one came with vehicle is working fine. Just buy and enjoy. However you must change all lubes and get its suspension done if it needs any work at all