A FULL playfield overlay looks good (sometimes) but will play SLOW. Most I've seen are rubbish. Some are so bad that you need floodlights for the inserts to light up. I'm not a fan at all.
A partial overlay - such as a mini playfield - I don't have as much as a problem as there are usually less lights and mini playfields are usually tucked away.
A Decal - Smaller areas where there is not enough damage for a partial overlay are fine.
Value of a full playfield on a machine ? My opinion is that they substantially decrease the value on titles such as KISS and Playboy - simply because these two games are valueable and most sellers would not mention that it is an overlay to trap unsuspecting buyers. I guess it is more acceptable on machines of lesser value.
Painting - More and more skillful pinheads are performing playfield restoration. Five years ago, you had to send them overseas. I'd post pictures of your project Rob. Let some of the knowledgeable guys form and opinion.