It appears that photo was part of a video. Are you able to post the video so we can see if it's fully working?
Williams machines are strong except for 2 things... Power supplies and the Dotmation screen. The photo you posted is actually what the machine would look like when the bottom power supply dies. Top screen turns on, rest of the machine is dead. There's a top power supply as well which is a bit more reliable than the bottom one, but there are tutorials posted in this forum on how to convert the power supply using a standard ATX computer one.
The Dotmation screen appears to be in good shape (bright, no outgassing, no missing lines). This screen is rugged but will degrade over time and eventually die, and it's already ~30 years old. Just something to keep in mind. There are no replacements available, and finding a working used one is like finding a unicorn.
EPROMs are easily enough to get to change the RTP %. Asking on here would be the easiest way. I'm not sure of the range Williams offered for this machine but I'd imagine it would be the standard 85%-98%. To change the RTP you'll need to replace 1 or 2 physical chips in the machine. You'll also need a Clear chip (again, easy to get) to make it happen.
The machine physically looks good, and if it's working 100%, I personally would be ok at $400 (for my area). Depends on the market in your area though.
It's a fun game, one of my favorites.