What is the condition of the backglass, playfield and cabinet? I think that will be the biggest consideration in terms of price. I know you mentioned the backglass is “pretty good” and the cabinet “had wear”, but that’s not a lot to go on. The electronics can be dealt with and, in my opinion, isn’t as nearly a big deal as condition. Just remember that a nice, working Joker Poker is about $1500 give or take. Maybe try to back into that price by adding up the cost of all the parts you think you will need plus a buffer for your time. Plus, you can always start low and react to a counteroffer.

I haven't seen a working system 1 mpu board in about 8 years. If the machine won't play a game, you can bet the main board is bad. New aftermarket boards are available, but plan on spending $300-400 and a lot of your time preparing the game for the new boards. The connectors will also likely need work, especially if the old battery was leaking. And if he didn't have the keys, I would assume it's leaking.

All that being said, it would be a great game to learn pin repair on as all the parts and info are available. If you posted a few pics, I'm sure someone could help you more with how much to offer.

Don’t worry about how long it will take you to do the repairs. Think of it as a journey and learning experience. It can be frustrating at times but very rewarding in the end. It looks like it can use a good cleaning, but it should clean up pretty well. I’d shoot for $500. It’s a desirable pin. Interested to hear what others say.

The other thing I’ll say is keep in mind people are desperate for project pins. If you don’t grab it, someone else will. If it’s a pin you want and you are up for the work, I wouldn’t worry too much about a couple hundred dollars either way. It might be a while until you find another project and there is value in saving the time to try to find another.

So what I've heard and experienced on system 1s is this:
The power supply probably needs rebuilding
The mpu might be good unless there is battery damage
Driver board is repairable
The interconnect and several connectors will need to be repinned
The small transformer needs a fuse.
The displays are usually good but might need to be burned in (easy) if they are dim

This is a desirable title and if it was in northern CA I'd buy it. But sys 1s take a specific startup sequence to test correctly before you can turn it on safely

So red flags:
Small transformer is melted or burned = run
Battery damage on mpu
Look for burned coils and manually push in the plungers on each one. Every burned coil is likely a driver board repair too