The risk with expired domain name is may be the domain is penalized from google before or had bad reputation. in case if you buy a expired domain and use it for your business so your domain can make your whole effort wasted. Once google penalize a domain that rare get lifted. In case if you buy a expired domain so you should become sure this domain is not penalized from google or search engines and had good reputation before.

Also seo factor is important. you should collect data using the backlink finder tool to know backlink of the site before. By redirecting urls to your own urls you can get benefit of previous made backlinks and that can lift your ranking so high also.