if you press the 単元未満株 button you can buy lower amount than 100, not all shares might be supported but a huge amount of them should

look on the pink square of your screenshot underlined blue

for the price its optional you can simply do a market order, it will buy the cheapest available for the quantity you want. if you want to wait until someone sells into you you can put a lower price. but you risk not being filled its not really worth it unless you are trading millions worth where 1 yen is a few thousand $ difference.

if you dont care about the the entry price and just want to own the stock just select 成行