The best software package is the one you use that adds value to your company.  The software that is best for one company is not necessarily the best for another.

What is the most important thing in selecting a software package?

There are two very important objectives in selecting a package: Obtaining user buy-in and avoiding surprises during implementation.  Involving users in the selection process and performing complete analysis of the package including a Scripted Demonstration  can accomplish both of these objectives.

How do I know if the sales person if telling the truth?

By checking and verifying the information.

Do I really need to go through the selection process?

Yes.  Even if the package you end up with is the same package you originally selected, the implementation will be faster and easier if you go through the selection process.  If you did not check out the package, there may be surprises during the implementation that will cause problems and delays.

Should I use one integrated package or a combination of packages (Best of Breed) that better meets my current requirements?

If the process will give you a strategic advantage, you should consider a Best of Breed solution.  To analyze the Best of Breed option, consider it as a vendor package and perform the R²ISC rating.  

Should I use a package or custom develop the programs?

If the process will give you a strategic advantage, you should use a custom developed solution.  Like the Best of Breed solution, consider it as a vendor package and perform the R²ISC rating. 

What does it mean if a vendor does not prepare for the scripted demonstration?

The package does not meet your current requirements or the vendor does not have time to prepare for the demonstration, which is a good indication of the assistance you will get during the implementation and the on going support you can expect.  An alternative is that the package is to hard to modify.  In any case be leery.

Should I reengineer my process before I look for a package or use the package to reengineer my processes?

The answer depends on whether or not the process will give you a strategic advantage.  If it does then reengineer before you look for the package.  If the process does then use the package as the bases for reengineering the process.