You will find that on-site needs of most of your clients is actually not that high, and most issues can be resolved remotely. Hardware is more reliable than it used to be, and business are increasingly buying hardware with replacement warranties or leasing.
So I would recommend installing a free program like logmein.com on all client PCs you will be servicing on an ongoing basis. This way you can provide quick response time, even when you are unable to be physically at a client's site.
Hope this helps.
