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For those items that you mention there, it would be cost prohibitive to replace the entire unit. Keep in mind, most of the newer toilets do not include the best seats either.
Chains are easy to swap out as are the seats.
Happy DIY'ing.
Call a plumbing professional. That way the toilets can be evaluated. You would not want to flush money down the drain on repairs if you are going to have to end up replacing the toilets in the near future. A plumbing professional would be able to go over all of your options, explaining the pros and cons of each. That way you can make an educated decision on what you should do.