I called listing agent and he said there is a 600K lien and mentioned a bathroom which was built without permits.

The thing about tear downs is that while you may get into the property ulta-cheap they will involve an awful lot of time and running around, meetings, permits, engineering etc. 

What permits would I need to apply for?

That fee is normal if he is managing the project  getting the permit  and doing the dawning you may be able to look over the bid and get a few items lower  when you look at it .Make sure you have it in your contract you have a right to remove a trade before they start and to use your person if you need to after looking at the bid and signing anything. 

Unless the parties agree otherwise in writing, Tenant may not permit, even temporarily, any pet on the Property (including but not limited to any mammal, reptile, bird, fish, rodent, or insect).B.

And the guy hadn't exceeded the two week maximum in the lease that guests are permitted to stay over.

Now make sure you only do this if you have a lease-option with the owner, rather than a regular lease, otherwise you won't be permitted to sell the property at all.

I did meet with a Keller Williams franchise owner and some statements seemed to suggest that I would be permitted to market under my own company name as long as the KW name, logo and colors were prominent; however there also seemed to be some uncertainty as to the exact rules and requirements / what's allowed / what's not.

I'd never try a furnace on my own without years of sinks and water heaters.The final note is always pull a permit and get a pro to sign off on work that needs to have their signature.  

If lot size permits I would plan on building a rentable mother-in-law suite at the back of the property.