On a budget, a I3-6100 will be perfectly happy with the cheapest lga1151 motherboard you can find. That will likely be a H100 chipset.
It likely will be a smaller M-ATX format which is no negative.
If you wish to prepare for the possibility of a future "K" upgrade and overclocking, you will need a motherboard with a Z170 chipset.
Post a link to where you will shop; I have no idea of your prices.
Cases are a personal thing, pick a couple of candidates that look good to you.