I have come across a site hat I want to buy which has decent rankings but not so decent link profile. The site seems to have lot of spammy links together with decent ones. If I buy this site is there a chance it could hurt my existing domains even if I do not link the sites in any way.
Google always reserves the absolute right to raise and lower rankings on any site for any reason. If you're sure you want to own a site which will need heavy backlink cleaning then I wouldn't worry too much about it affecting your other sites but, unfortunately, there is still a chance it does somehow impact your other site(s) even if it's a negligible one. If you're comfortable buying and cleaning up a spammy site to begin with then you're probably comfortable with the small risk as well.