This sounds like it could be classed as configuration data. This msdn article - Managing Application Configuration - describes the options available to you.
Basically you can store the properties in either:
- web.config
- hierarchical object store
- property bag
- list
I'd therefore recommend a list which is secured so only certain people can access it.
However, if the settings aren't site wide, those will prove problematic as you won't know how many times the web part is used and therefore won't be able to easily create a storage place for each instance.
I would therefore recommend implementing some property validation to stop invalid values, for example:
public string text;
[Category("Advanced Settings"),
Personalizable(PersonalizationScope.Shared),
WebBrowsable, WebDisplayName("Text"),
WebDescription("Enter your text")]
public string Text
{
get { return text; }
set
{
// Validate input
if(value.Length < 10)
throw new WebPartPageUserException("Enter minimum 10 charectors in text field.");
text = value;
}
}