1. You absolutely need an LLC in a partnership business in my opinion and personal experience. You need to be mindful of the articles as well, and tailor them to you and your business partner(s) - for example, what if your partner wants to sell some of his "shares" to a third party? Is that allowed? Think about all the "what ifs" that could go right or wrong, and figure out how you want the business legally structured. I would avoid any partnership that is not equal shares, because it can go sideways very quickly in that arrangement.
2. The bank will want to see you and your business partner(s) tax returns and personal financial statements. However, any and all "seed money" needs to be a capital contribution to your LLC. Keep ALL monetary items closely recorded, and absolutely separate - aka do not mix business and personal accounts whatsoever going forward once the LLC is set up. If you contribute money to the LLC, it must be done equally with your partners and recorded as capital contributions for tax reasons.
3. We set up our own spreadsheets and analysis. There are many others on this site who can answer that question much better than me, as I don't do nearly as much analytics as the full time investors here.
4. A word of caution - there is a big difference between a "friend" and a "business partner". Be aware there is a reasonable chance your friendship could end if things go wrong. If you are transparent and honest with each other from the day of LLC inception on ALL money matters, you have the best chance of success and preserved friendship. Good luck.