I have a list of must-buy mods when I get a new game... Flipper Fidelity speakers, LED OCD, BriteButtons, and speaker lights.
On to toppers... I love toppers. I think they round out the look of the game, as long as they are professional looking, i.e. Tilttopper, laseriffic, Stern, etc.
I’m not a big fan of PDI glass. I don’t see (pun) the value, but it seems like I’m in the minority.
Having said all that, ColorDMD (if available) is where the money goes first.