What is the physical/electrical limit of audio quality?
What is the maximum physical/real/electronic bitsize in which a piece of audio can be stored?

As Dzarda comments, this is not a sensible question, and it is not clear what you mean by 'piece'. If you mean sample, you can store it in as many bits as you can store. Typical HDs contain 1 TB and more, so 8 Tera Bits would be within reach.

will 32bit audio be overkill/contain too much noise?

It is overkill in the same way that it makes no sense to protect your bike with a very heavy chain that is closed with a soft plastic padlock. You'd better spend less money on the chain and use it to buy a better padlock.
Let's for the sake of argument say that the signal/noise ratio from the analog parts of your audio system corresponds to 16 bits. If you play back digital sound stored as 18 bits that adds ~3% of that level worth of noise: it increased the noise by ~3%. (from 100 to 125, in arbitrary units). 20 bits will increase it by 0.7%. 32 bits by 0.00098%. That is: assuming you have a perfect translation from digital to analog.
The cost of storage increases linearly with bit size, the cost of a full-range-accurate D/A converter raises almost exponentially when you approach a certain number of bits (~22?). So using more bits than the equivalent quality in the analog parts costs more, but the gain in quality diminishes. So it is simply not economical to use more bits: if you want to spend more money to get better quality, you should spend it on the analog parts. (I am not a audiophile, but AFAIK the speaker is often the weakest link.)
This is a common theme in engineering: it is not about doing individual parts as good as possible, but about a balanced design.