as for the design itself, is the initial version of the preev.com site really that good, so most similar services copy it?
Yes, you can't go wrong with an efficient and minimalistic approach.
is it really necessary to have all possible currencies, for example, 170 FIAT and 5k+ cryptocurrencies? Or top 10 or 20 from CMC is enough?
Considering that there are certain countries with dual/multiple exchange rates, I don't think it'd be a good decision to include a lot of them
[the top 5 most traded currencies will be more than enough] and as for the cryptocurrency part, I prefer to go with
BTCitcoin only.
do you keep a page like this open all the time, to keep track of the current value? which update timer is desirable in that case?
I do, but it doesn't have to display the value in real-time
[anything that doesn't go beyond the minute interval is the sweet spot for me].
I saw some interesting features like the historical price that were liked by some members. whether such things are necessary?
It could come in handy for some users, but I prefer to not disrupt its current design.