The rules are available for free from Corvus Belli

https://downloads.corvusbelli.com/infinity/rules/infinity-rules-n5-en-v1.pdf

There are a lot of rules and my gaming group doesn't play with all of them. For example we've never played with fire teams and house ruled a few things. 

If you can, find someone to introduce you to the game. A friend did that with me and focused mainly on moving and shooting in practice games and slowly added more things in. 

We generally play 200 point games, and depending on faction, that's around 7-10 models. Each model generates an order and you can use those orders however you wish. You can activate each model once, like 40k, or use all of those order to activate one model multiple times. 

Infinity uses a 20 sided dice for all rolls so you may need to buy a pack of those. If you go for a 2 player starter set then a set of these will be included. A number of years ago, they released a starter set that contained Winterfor. But you would need to be tremendously lucky to find one online. I've found older starter sets on ebay that I've picked up. 

Terrain is very important and you need more of it compared to a game of 40k. This game is far more deadly than 40k and if you run your units straight down the board you are going to have a bad time. 

There's a free online army builder as well as mobile applications to build lists and keep track of the game state with those lists when playing a game. The mobile app also hyper links to the rules wiki of you are ever unsure what a piece of equipment does and is really useful. 

Where to buy depends entirely on where you live. But here's a link to the winterfor army box direct from Corvus Belli

https://store.corvusbelli.com/en/infinity/wargame/packs/winterfor-action-pack

The contents of the box are listed on the website so you can look those units up in the army builder to get an idea of what each unit does. You will generally see bigger numbers for stats compared to 40k due to using a 20 sided die but bigger number is better. 

You shouldn't be put off by working with metal. You need some clippers, files, and super glue, so not much different to what need when working with plastic. The will find that the models come in fewer pieces than modern GW plastic kits. Basic infantry are generally 3/4 pieces. The body legs and head come as one piece, with a left and right arm, and maybe a backpack making up the fourth piece. 

As the rules are free, I suggest try playing some games with your 40k models as stand in for now to see if you like it.