There is often the option of producing a hybrid betweeen bespoke and packaged software. In this case, for example, if you needed to perform a mail-merge with a specialist printing machine, you could use a common package to generate the list of recipients and bespoke software to connect to the specialist machine.

How do I decide?
The starting point has to be an understanding of what you want to achieve. Go through the process of specifying your requirements, and then look round at the packages which seem to match this need.

Is there a package sufficient for you?
If it is, then you should probably buy it.

Does a package do much of what you want?
Investigate whether it really is a close match. For example, US Sales Tax is very different to European Value Added Tax and you would do as well to disregard this as a match. Can you extend it to meet your needs? Maybe you just want a couple of reports adding, but maybe you need some other things changing too. Some vendors charge highly for modifications to their system, and you might be unable to upgrade to the next version at the same price as everyone else.

Can't find anything suitable?
Then you'll need to go bespoke.

Are the commercial opportunities of specialist software appealing?
Then you'll have to consider bespoke.

You should consider getting in some specialist assistance if the costs are going to be more than perhaps £1000- we are most willing to provide an initial free consultation to check that you are taking the right course.

Concentrate your efforts on the software that will give you a strategic advantage- don't write your own word processor software!