Imo you must know the business and sector back to front eg product or service knowledge required and the customers involved,or love doing the work required preference would be a combination of both and this would imo be a good start to being able to value the business.Good luck finding that business for sale.
ps i probably agree with jantimont my preference would be to start from scratch but your question is obviously very broad.
Gutsy decision in this environment,because i'm trying to think of a business sector i would want to buy into at the moment,maybe agriculture eg specialised farming with vertically integrated finished product has always appealed to me since i sold my manufacturing business.
