It is simple,. .IF you know what your needs are (cabin,runway,range,fuelcosts,purchase etc.). .When you buy,use a citation center to do a prepurchase inspection, read the cescom maintenance file very carefully (cescom is an option but most operator use it) to find out about whats next on the list. Add the rvsm to your calculation and be aware that aeroplanes very often show a nasty behavior when beeing sold (somethings breaks right after the purchase, every airplane our company bought proofed this). Tradings are slow these days, should be easy to get a good piece of aluminum!