I'd like to mark plastic or metal permanently with a custom design.
The ink needs to dry quickly and not smear. Stamps made need to
withstand mild cleaning with water or solvent cleaners.

1. What type of stamps would work best for me, the self inking style
rubber style, the rubber stamp & pad sort, the sort that absorb ink?
Why?

2. What type of ink fits my needs?

3. Where can I buy on the web all I need, the custom designed stamps
and the ink for a reasonable price?

Most of the so called Pemanent ink pens will permenantly mark most
plastics or metals. The typical Lumicolor pens used for drawing on OHP
are good permenant markers. I am not sure if they also do ink pads but
syspect they do.

A lot depends on the actual plastic or metal surface in question but
if you start with these types of pemenant markers you wont go far
wrong. I suse them all the time to mark a range of plastic and metal
surfaces.

There's "STAZON Ink Pads & Reinkers: Acid-free, Archival, Fast drying
solvent. Designed especially for use on plastic, metal, glass,
ceramic, laminated or, coated paper and leather. Dries in 3-5 minutes
on non-porous surfaces like plastic." from this website,
http://www.blockheadstamps.com/ink.html.

You might also find this site interesting, http://www.stampin.com/tips/popcan.htm.

Hope this helps