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I wouldn't even consider selling a car for another instead of adding power steering, unless is was an ultra rare, extremely high value car. Plenty of choices out there for adding power steering.
If you can find all the parts to convert to power steering, and you consider it to be 'affordable', go for it. There are conversion kits out there. Borgeson boxes are relatively expensive and basically nothing more than putting a Jeep P/S unit in your car. Do good research to determine what system will best meet your needs.
Unless I really didn't have any 'love' for my car, I wouldn't want to sell/buy to get one with P/S. Sounds like a lot to go thru for just that. If there are other C3's that you would prefer to buy, anyway, just make sure the next car has P/S.
Just for discussion... The only downside to having a non-P/S vehicle (IMO) is the difficulty in trying to steer the car when at a dead-stop. If you get the car moving a bit (even creeping), steering is much more manageable. At least, that has been my experience. Another option () might be to hit the gym and beef up those arms!!
I had a shop add PS to my '69 eighteen months ago. Made it so much more enjoyable to drive. Probably cost around $4K total, but rebuilt valve is leaking already
I added stock power steering to my 68 ages ago and it became much more pleasurable to drive. If I were to do it today, then I would go with the Borgeson conversion.