I've looked at lot of options and have found the least expensive is to have the bathroom sink within reach of the toilet. Just wet toilet paper and use.
The non-heated units aren't worth the money. They are either too cold, or you have to let them run too long to get the right temp.
I hate to say it, but cold water does NOT feel good.
The water pressure changes is not a problem.
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The best compromise seems to get a ~$300 seat with a small heated tank and add a tempering valve (if needed) so that you send warm water to the unit to prevent it getting cold during use.
Edit: This works best if you are willing to put a hot circulating loop in.
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If you do go with a non-heated unit, remember that you will need a tempering valve that can keep the water close to the 98F desired temperature.
Then keep in mind that you will have to dump enough water to get warm water to the head before use.
Edit: This works best if you are willing to put a hot circulating loop in.
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Edit again:
http://terrylove.biz/12-bidet
Scroll to bottom: Heated toilet seat by Takagi, elongated
I can't see any specs as to how much water it holds or much of anything else.