I was approached by a seller who is interested in selling their property but claims it will have to be torn down. How would I go about estimating the MAO I could give them?

I would chop that up to routine maintenance / wear and tear and get the boards replaced. 

Sure, you can order HD windows which will do the trick but they're definitely entry level and will not sustain with the wear and tear of island wind and sea water damage.

NO - you unfortunately cannot unless the tenant has caused unusual wear and tear, regardless of the lease statements.

the tenants have also damaged the floors beyond normal wear and tear as well as damages made on walls such as painting etc and damages to countertops then does this mean they are responsible for the costs of me hiring someone to repair those things?

Bonus is 2 ADU's in the back (possibly makes sense to do a tear down and build new on that square footage) but there are 6 doors on property.I was supposed to get renovation money (rehab loan) but looks like lender messed up that deal.

Even a traditional construction loan requires a big down payments, so buying the tear down with cash might make the most sense.  

I currently have a property under contract that has a strong potential to be a tear down as it is situated right in the middle of 2 new luxury home builds.

Others chew wood trim or scratch up doors or tear down blinds.

If he wants to play remodel, let him tear his own place apart.