Dive into anything

Hello!

Introduction: This is my first motorcycle ever - I am not into cars or motorcycles usually and this is my starter bike to learn to ride on. I got the cheapest thing I could find over Facebook marketplace and the guy drove it to my place.

I figured there might be some issues - I look at it as an opportunity to learn to work on motorcycles as this might be my new hobby. Keep in mind that I am not savvy when it comes to terminology but will do my best to understand.

Problem: There is a leak from where countershaft is (I am not sure if correct term is crankshaft or countershaft) but where there is a front sprocket with an oil seal. There seems to be a washer (crankshaft oil sealing) that is damaged inside there behind a large nut and that's where the leak is coming from.

Question: So I have reached the area as you can see in the photos below. My question is What do I need to buy and from where - looking at Suzuki Parts website for Crankshaft diagram do I need #13 (Washer that is damaged) and #12 (Oil Seal) and that's it? What else is recommended to be changed during this type of work? Can anything else be damaged that I should consider buying new to replace? Also should I buy a new sprocket? What about the chain? it has quite a bit of gunk and oil on it , do I take it off and clean it or do I buy a new one?

Also where should I buy from? Is buying from Suzuki parts the way to go? Is there another website that can offer better pricing/faster delivery?

Photos : Any help will really be appreciated on my next steps, what to buy, from where and any other tips you can give me! ( Oh I also need to find out how the heck will I be taking off that big nut if there are any tips on that one )

Photos here : https://imgur.com/a/Q5r1SpY