First off, you should use a server; a machine that does nothing but handle the database, core application, and system utility and authentication / authorization functions.
When you put the core system on one of the work machines, you crank up the risk significantly that something will eat your database. The POS machines are exposed to the public, which also increases the risk.
Put the server in a secure location ("Physical security").
Database apps, even relatively simple ones, require a lot of resources (ram, disk, and processor).
What sort of backup are you planning to use? Have you figured out the cost of having a down system versus spending a little more up-front to get a better, more reliable system?
If you assume responsibility for design and implementation, you also assume the liability. Do you have insurance to cover the possibillity that something will not work as planned and maybe ending up in court?
POS systems, in general, are fairly complex systems. The advice to bring in a POS consultant is solid advice. Any decent consulting service will be bonded and insured. The extra money you put into using a consultant is usually money saved in the long run (i.e., they have already bled, and can often avoid the less-obvious potholes of implementation and design).
Good Luck
Scott