I'm a huge cruiser proponent and have been for many decades. I own a couple and will never be without one.

That said, cruisers are very bad hill bikes because of the riding position. The modern "crank forward" frames like the Electra are even worse. Cruisers are designed for slow, leisurely, comfortable, low effort riding on flat land. They do that job wonderfully. Climbing hills while seated will suck, regardless of the gear ratios available. And climbing out of the saddle is horrible on a cruiser.

My recommendation is don't do it.

If you chose to go for it the costs associated with the project will partially depend on whether you do the work yourself or have your LBS do it. You'll have to buy an internally geared rear hub and new spokes, completely disassemble and reassemble the rear wheel. Or, buy an entire rear wheel already assembled. Then there is mounting the shifter, and routing, cutting, crimping, installing the cable.

If the bike has a coaster brake you will lose your rear brake and need to buy a rear or front wheel hand brake. Meaning more shopping, brake selection, lever selection, cable work, pad adjusting etc.

Then there is the frame spacing at the rear drop outs. It is probably 110mms for it's current hub. Internally geared hubs have a wider Over Locknut Dimension. So you will need to "Cold Set" (bend) the frame (rear triangle) to accommodate the bigger hub. You'll need to be able to do this while maintaining accurate frame alignment. Or, you'll have to pay a shop to do it.

If you do the work it'll be $$$, if you have a shop do the work it'll be $$$$$$.

This why I don't understand the whole multi-speed cruiser thing. All of the above can be done. I've done it. But what you'll end up with is a tool that does it's job poorly.

My recommendation is to look for a hybrid. They have upright comfortable riding positions, multiple gears, and most will accept large tires for a comfy ride.