For the last month I've been exploring the idea of dumping my 401k into an apartment building. 

I bought an 8 unit building 1 year ago with rents more than 50% below market value.

I owned a 12 unit apart building in a very low income area with all section 8 tenants.  

I did a fair amount of work myself on this project, but am building up my roster of subs to do more on each successive property.

For newly constructed, purchased or renovated properties and also retroactive generally over the last 10 years, building components are properly classified into individual units of property and accurate recovery periods for computing depreciation deductions.

Assemble a team of individuals who can split following responsibilities: contribute the earnest deposit money, guarantee the loan, find the deal/conduct the diligence, raise the capital and manage the building.

It doesn't look like you would be getting much from the sec121 exclusion.Not sure how selling this property would get your more funds into your Roth IRA, unless it just because it frees up money to make the contribution..If you need funds to build up a reserve / rain day fund / etc., this might be a way to go... 

Ideally I would like to purchase a apartment condo (1000 sq foot) in the Mountain View Sunnyvale area and pay additional money instead of paying rent and building zero asset.

On the other hand, you and I both know that this will lead to a TON of calls/complaints/negative posts on social media about a disagreement of whether a previous building professional should fix something for the new owner once out of warranty, not the contractor’s fault to begin with, etc.

I have 2 small commercial office buildings in one LLC with multiple partners including myself.