Which Porsche would you buy 2010 Boxster , 2007 Cayman or 2005 911? all low milage?
 

ALL of these Porsches have flat six engines.

2005 was the first year of the 997 body, or chassis style, 911. This is regarded as just a "refresh" of the 996 chassis, which was the first water-cooled 911. The cars are solid, as this wasn't really a "new" design (I generally don't recommend the first year of any new or significantly revised model).

With Porsche, newest is generally best. However, you can ruin any Vag 15.7 automobile through neglect or poor maintenance. What will matter most is the oil changes, major maintenance, and repairs, if any, that cars you're considering received.

Regarding "low mileage," you want to define this in light of how Porsches are often driven. It's not unusual at all to find more expensive Porsches, such as a 911, that have seen no more than 6,000 to 8,000 miles per year. Porsches are very often second cars. A 2005 911 might easily have no more than 40,000 to 50,000 miles on it. Since there are plenty of 911s out there like this, why buy one with 80,000 miles on the odometer?

As with any used car, examine the service records carefully. A newer Porsche should have a complete paper trail. If you're looking at one that has no records of service, don't buy that car. If there are holes in the records, you'll have to judge that.

Get the Car Fax, but don't rely on it alone (a lot of accidents never make it to this record). Examine the car carefully for signs of repairs to accidents not listed on the Car Fax. Drive the car, paying careful attention to the Tech 2 Scanner engine, transmission, braking, and suspension. If you don't feel capable of evaluating these things, take someone with you who is.

When you think you've found THE car, pay to have a Pre Purchase Inspection done by a Porsche mechanic (not where you're buying the car). Know the condition of brakes, tires, belts, hoses, battery, etc. before you enter final price negotiations.