I was approached by somebody who wants to set up a regional center regulated and monitored by US Immigration Service &SEC.

I live in California where the Democrat policies have been in full force for 60 years, with massive government funded programs that give our tax dollars to pedophiles, ex cons, drug addicts and alocoholics under the guise of disability, free educations and priority to illegal immigrants, government unions demanding unsustainable benefits and wages, higher taxes a constant theme on our ballots, environmental regulations that require 8 different grades of gasoline throughout the state (which raises prices), hundreds of thousands of acres of farmland that cannot be farmed due to the Dems putting water for smelt over farmers, car registration fees so high that many lower and mid income people (especially the unemployed) are losing their vehicles, a community college system that is so top heavy it now takes 4 years for kids to finish a 2 year program for transfer to a university, and much much more.

In bigger cases, even with immigration enforcement. 

Typically first generation immigrants are very, very fearful of the new community that they are moving into.

I am considering buying a commercial property in Colorado and am wondering if I'll have to honor a lease that is with an illegal immigrant. 

Maybe there is a new immigrant population that has flocked to the area.

Almost of those younger inhabitants are white and quite a few of them have anti-immigrant views.

My wife and I are both 1st gen Americans from Slavic immigrant parents and grandparents.

I think your renter base will mostly be immigrants. the area is near white center which is mostly a C- or D hood.

If you really want to get accurate with your return then take ordinary income tax from the $15K which including self employment tax will run you almost $7K so you'll end up with an $8K profit at best.And  yet even more potential bad news - Depending on your immigration status you will be  subject to FIRPTA withholding of 15% of the sales price of the new property.