Agreed - don't give yourself a deadline. Also, be conscious of costs, but don't set a strict budget (things will go wrong and parts will have to be re-bought or re-made. Don't let it get you down).

I re-purposed a cabinet and playfield, which saved probably a year of work and shaved at least $1000 off the supply budget.

Most importantly, start with the end in mind. Before starting anything, visualize how you want the game to look, feel and sound. Write out lists and draw diagrams of how the game modes should work and progress. Sketch some artwork. Think about what you do and don't like on existing games.

Lastly, before you begin, identify people who can help with electrical and programming issues and with artwork and printing. I could not have made Queen without the P-ROC and MPF online forums.

I started with a RasPi B+ and quickly discovered that the processing power is way too slow for pinball. I ended up with an Asus Tinkerboard - same footprint, twice the power and speed.

Along the same lines, give yourself more cable than you think you need leading from the playfield to the backbox. 6' is barely enough - give it 8 or 12 to be safe. You don't want to keep yanking the connections out every time you lift the playfield. Being stingy with wire is one of my regrets on Queen.

Since my project used an existing playfield, I had to think up a use for every insert - I had to use all of them and couldn't drill any new ones. For example, my concept didn't include bonus scoring, so the bonus lights became a mode progress meter and the two bonus multiplier inserts became playfield scoring multiplier indicators. A row of three inserts on the right side are only used like runway lights, directing the player to lock the ball.

I used MPF and P-ROC hardware and a Raspberry Pi 3 was too slow. Even without elaborate display graphics, there's a ton of stuff all going on at once - sounds stack up and a game can run a dozen or more sub-routines and timers all at the same time. It gets more complicated than you may suspect.