1) Insist upon an LLC. in that document you can define the goals and % interest of the parties. The LLC should be the borrower.
a) some lenders don't like this-- especially if you insist on a non-recourse loan. if you have decent assets or have done multiple deals-- even in Residential SFRs-- then you should already be aware of this tidbit-- if not, it's a fight for your first one.
2) Not an easy question. I see you have an offer in the thread for PM- take (someone) up on that. Maybe it's someone here, maybe it's someone from a local REIA, but get someone's take on your numbers. There are many ways to skin this cat-- the trick as a borrower is to show the lender the value they need to see so they don't artificially depress your values. Remember: the bank has one goal-- to mitigate its risk.
3) your LLC doc mentioned in (1) should describe your % share, and what you're bringing to the table to justify this share. Sometimes it's capital, sometimes it's experience, sometimes it's hound-dogging. agree on the tenets of this question in (1) and everyione should sign and agree to your operating agreements. In many cases, the lender will want to see those docs too-- don't be surprised. in some jurisdictions you can push back and say no... but they can also say 'no' to your loan.
4) Hire a lawyer. Let them worry about title, Enviro, and Zoning matters. Hire an accountant, and especially when you're new to the asset class-- HIRE A BUILDING INSPECTOR who is used to working in this class of properties. Too many investors go without the inspection-- short term money for long term liability. an inspection gives you an objective third-party benchmark about the condition of the property from which you can re-negotiate your purchase price. Lots of sellers will tell you to buzz off-- those are exactly the ones you need to watch out for. At the very least, have a Construction Supervisor or GC on your team.
Also this: a trip to the Town or County office to inspect Zoning, Building Inspection, and Planning records should ABSOLUTELY be part of your DD prior to your offer. This is three different departments or boards in most jurisdictions. If you see a new kitchen and no permits... well, now you know. Again, it gives you some leverage to negotiate from. Furthermore, it's a much higher risk that something will be wrong with multi families-- there are stories every month of someone buying a (4-family) and then a month after the purchase learning it's only legal for (2 families.) NOTHING will turn your math upside down like a reduction in total legal units. Search "Illegal apartment" or "only X units allowed" in the forums here, and soak in the wisdom on those threads.
5) Not really-- every property is different. Some need zoning relief, for example, and some don't. Some have pools, some have basements, some have electrical issues. There are lots of base-level checklists out there, but they won't cover everything.
Good luck.