It is a great way to build a retirement and risks can be reduced. 

Can I get 80% from the lender, 15% from the seller, and put down 5% myself to buy the building?

I am an agent and trying to find an off market buyer for a large commercial/apartment building for my broker.

One strategy I like to do is build in seller credits into my offers so the loan covers the closing costs, e.g. if you wanted to offer $500K for a property, offer $510K with $10K towards closing costs and that $10K is supported by the loan and can be used on the costs I mentioned above.

That way I could likely get the work done for less and hopefully build a long term partnership with someone that is looking to get their foot in the design world.

It sounds like you've made great progress in building your real estate portfolio, especially with the support of your grandfather.

It's a testament to the power of experience, relationships, and sound financial strategies in building a successful investment portfolio.The plan?

Perhaps the biggest opportunity for mishap in this area is when going from a building that has been cost segregated and wanting to 1031 into land. 

Keep all the units I currently own and just save up for each additional unit like I have done to build up what I currently own?

I think you should be a bit more confident in the deal based on the rents and purchase price.